U.S. patent application number 15/782066 was filed with the patent office on 2018-06-07 for self-suspending proppants.
The applicant listed for this patent is SELF-SUSPENDING PROPPANT LLC. Invention is credited to Moustafa Aboushabana, James Nathan Ashcraft, Kanth Josyula, Vinay Mehta, An Thien Nguyen, Allison Silverstone, David S. Soane, Huaxiang Yang.
Application Number | 20180155614 15/782066 |
Document ID | / |
Family ID | 60191496 |
Filed Date | 2018-06-07 |
United States Patent
Application |
20180155614 |
Kind Code |
A1 |
Soane; David S. ; et
al. |
June 7, 2018 |
SELF-SUSPENDING PROPPANTS
Abstract
A self-suspending proppant comprises a proppant substrate
particle and a water-swellable composite coating on the proppant
substrate particle comprising the combination of at least two of an
anionic hydrogel polymer, a cationic hydrogel polymer and a
nonionic hydrogel polymer.
Inventors: |
Soane; David S.; (Chestnut
Hill, MA) ; Aboushabana; Moustafa; (Stafford, TX)
; Ashcraft; James Nathan; (Jupiter, FL) ;
Silverstone; Allison; (Hillsboro Beach, FL) ;
Josyula; Kanth; (Sugar Land, TX) ; Yang;
Huaxiang; (Sugar Land, TX) ; Nguyen; An Thien;
(Sugar Land, TX) ; Mehta; Vinay; (Richmond,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SELF-SUSPENDING PROPPANT LLC |
Chesterland |
OH |
US |
|
|
Family ID: |
60191496 |
Appl. No.: |
15/782066 |
Filed: |
October 12, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62407611 |
Oct 13, 2016 |
|
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|
62428258 |
Nov 30, 2016 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 43/267 20130101;
C09K 8/685 20130101; C09K 8/887 20130101; C09K 8/805 20130101 |
International
Class: |
C09K 8/80 20060101
C09K008/80; C09K 8/68 20060101 C09K008/68; E21B 43/267 20060101
E21B043/267 |
Claims
1. A process for fracturing a subterranean geological formation
comprising introducing into the formation an aqueous fracturing
fluid containing an aqueous carrier liquid and a modified proppant
comprising a proppant substrate particle and a hydrogel polymer
coating on the proppant substrate particle, wherein the hydrogel
polymer coating comprises the combination of a cationic
polyacrylamide polymer and an anionic polyacrylamide polymer, and
further wherein prior to reaching its final destination downhole
the modified proppant is exposed to a level of water hardness which
is sufficient to adversely affect the ability of said anionic
polyacrylamide polymer to swell.
2. The process of claim 1, wherein prior to reaching its final
destination downhole the modified proppant is exposed to a level of
water hardness of at least 300 ppm.
3. The process of claim 2, wherein the aqueous carrier liquid from
which the aqueous fracturing fluid is made has a level of water
hardness of at least 300 ppm.
4. The process of claim 2, wherein the modified proppant encounters
geological formation water prior to reaching its final destination
downhole, and further wherein the geological formation water has a
level of water hardness of at least 300 ppm.
5. The process of claim 2, wherein the hydrogel polymer coating
comprises about 70 to 90 wt. % cationic polyacrylamide polymer and
about 10 to 30 wt. % anionic polyacrylamide polymer.
6. The process of claim 5, wherein the modified proppant is made by
(a) forming a premix of a cationic polyacrylamide polymer invert
emulsion and an anionic polyacrylamide polymer invert emulsion, (b)
combining the premix so formed with the proppant substrate particle
with mixing, thereby forming a polymer/particle mixture, (c)
continuing to mix the polymer/particle mixture until the hydrogel
polymer coating is formed, and (d) drying the hydrogel polymer
coating.
7. The process of claim 6, wherein the hydrogel polymer coating is
crosslinked by means of a covalent crosslinking agent.
8. The process of claim 7, wherein the proppant substrate particle
is coated with a first covalent crosslinking agent before the
polymer/particle mixture is formed and further wherein a second
covalent crosslinking agent is combined with the polymer/particle
mixture before the hydrogel polymer coating is dried.
9. The process of claim 5, wherein the hydrogel polymer coating is
made by (a) combining the proppant substrate particle with a
cationic polyacrylamide polymer invert emulsion to form a first
polymer/particle mixture, (b) combining the first polymer/particle
mixture so formed with an anionic polyacrylamide polymer invert
emulsion to form a second polymer/particle mixture, (c) continuing
to mix the second polymer/particle mixture until the hydrogel
polymer coating is formed, and (d) drying the hydrogel polymer
coating.
10. The process of claim 9, wherein the hydrogel polymer coating is
crosslinked by means of a covalent crosslinking agent.
11. The process of claim 10, wherein the proppant substrate
particle is coated with a first covalent crosslinking agent before
the first polymer/particle mixture is formed and further wherein a
second covalent crosslinking agent is combined with the second
polymer/particle mixture before the hydrogel polymer coating is
dried.
12. The process of claim 5, wherein the hydrogel polymer coating is
made by (a) combining the proppant substrate particle with an
anionic polyacrylamide polymer invert emulsion to form a first
polymer/particle mixture, (b) combining the first polymer/particle
mixture so formed with cationic polyacrylamide polymer invert
emulsion to form a second polymer/particle mixture, (c) continuing
to mix the second polymer/particle mixture until the hydrogel
polymer coating is formed, and (d) drying the hydrogel polymer
coating.
13. The process of claim 12, wherein the hydrogel polymer coating
is crosslinked by means of a covalent crosslinking agent.
14. The process of claim 13, wherein the proppant substrate
particle is coated with a first covalent crosslinking agent before
the first polymer/particle mixture is formed and further wherein a
second covalent crosslinking agent is combined with the second
polymer/particle mixture before the hydrogel polymer coating is
dried.
15. The process of claim 5, wherein the modified proppant exhibits
a volumetric expansion of at least about 1.3 after having been
subjected to shear mixing in a simulated hard water containing
6,400 ppm hardness at a shear rate of about 511 s.sup.-1 for 10
minutes.
16. A process for fracturing a subterranean geological formation
comprising introducing into the formation an aqueous fracturing
fluid containing an aqueous carrier liquid and a modified proppant
comprising a proppant substrate particle and a hydrogel polymer
coating on the proppant substrate particle, wherein the hydrogel
polymer coating comprises the combination of a starch and either a
cationic polyacrylamide polymer or an anionic hydrogel polymer, and
further wherein prior to reaching its final destination downhole
the modified proppant is exposed to a level of water hardness which
is sufficient to adversely affect the ability of said anionic
hydrogel polymer to swell.
17. The process of claim 16, wherein prior to reaching its final
destination downhole the modified proppant is exposed to a level of
water hardness of at least 300 ppm.
18. The process of claim 17, wherein the aqueous carrier liquid
from which the aqueous fracturing fluid is made has a level of
water hardness of at least 300 ppm.
19. The process of claim 18, wherein the modified proppant
encounters geological formation water prior to reaching its final
destination downhole, and further wherein the geological formation
water has a level of water hardness of at least 300 ppm.
20. The process of claim 16, wherein the hydrogel polymer coating
comprises the combination of a nonionic starch and a cationic
polyacrylamide polymer.
21. The process of claim 16, wherein the hydrogel polymer coating
comprises the combination of a hydrolyzed starch and an anionic
hydrogel polymer.
22. The process of claim 16, wherein the modified proppant exhibits
a volumetric expansion of at least about 1.3 after having been
subjected to shear mixing in a simulated hard water containing
6,400 ppm hardness at a shear rate of about 511 s.sup.-1 for 10
minutes.
23. A modified proppant comprising a proppant substrate particle
and a hydrogel polymer coating on the proppant substrate particle,
wherein the hydrogel polymer coating comprises the combination of a
starch and either a cationic polyacrylamide polymer or an anionic
hydrogel polymer.
24. The modified proppant of claim 23, wherein the hydrogel polymer
coating comprises the combination of a nonionic starch and a
cationic polyacrylamide polymer.
25. The modified proppant of claim 23, wherein the hydrogel polymer
coating comprises the combination of a hydrolyzed starch and an
anionic hydrogel polymer.
26. The modified proppant of claim 23, wherein the hydrogel polymer
coating is crosslinked by means of a covalent crosslinking
agent.
27. The modified proppant of claim 23, wherein the modified
proppant exhibits a volumetric expansion of at least about 1.3
after having been subjected to shear mixing in a simulated hard
water containing 6,400 ppm hardness at a shear rate of about 511
s.sup.-1 for 10 minutes.
28. A process for fracturing a subterranean geological formation
comprising introducing into the formation an aqueous fracturing
fluid containing an aqueous carrier liquid and a modified proppant
comprising a proppant substrate particle and a hydrogel polymer
coating on the proppant substrate particle, wherein the hydrogel
polymer coating comprises the combination of a cationic
polyacrylamide polymer or an anionic polyacrylamide polymer,
wherein the amount of the anionic polymer to total polymer is less
than about 50 wt % on a dry weight basis, the amount of the
cationic polymer to total polymer is at least about 50% wt % on a
dry weight basis and the hydrogel polymer coating is crosslinked by
means of a covalent crosslinking agent.
29. The process of claim 28, wherein the aqueous carrier liquid
from which the aqueous fracturing fluid is made has a level of
water hardness of at least 300 ppm.
30. The process of claim 28, wherein the hydrogel polymer coating
is made by (a) combining the proppant substrate particle with a
cationic polyacrylamide polymer invert emulsion to form a first
polymer/particle mixture, (b) combining the first polymer/particle
mixture so formed with an anionic polyacrylamide polymer invert
emulsion to form a second polymer/particle mixture, (c) continuing
to mix the second polymer/particle mixture until the hydrogel
polymer coating is formed, (d) adding a covalent crosslinking agent
and (e) drying the hydrogel polymer coating.
31. The process of claim 28 wherein the covalent crosslinking agent
is polymeric methylenediphenyldiisocyanate.
32. The process of claim 28 wherein the amount of the anionic
polymer to total polymer is less than about 30 wt % on a dry weight
basis, the amount of the cationic polymer to total polymer is at
least about 70% wt % on a dry weight basis.
33. The process of claim 30 wherein the proppant substrate is
pretreated with a solution of polyethylenediglycidyl ether prior to
combining the particle with a cationic polyacrylamide polymer
invert emulsion.
Description
RELATED APPLICATIONS
[0001] This application claims priority to provisional application,
U.S. Ser. No. 62/407,611, filed Oct. 13, 2016, entitled Amphoteric
Self-Suspending Proppants by Moustafa Aboushabana, et al.,
(Attorney Docket No. 17922/05187) and to U.S. Ser. No. 62/428,258,
filed Nov. 30, 2016, entitled Self-Suspending Proppants by David S.
Soane et al. The contents of both applications are incorporated
herein by reference in their entireties.
BACKGROUND
[0002] In commonly assigned U.S. Pat. No. 9,297,244 (7-US) and U.S.
Pat. No. 9,315,721 (4-US), there are described self-suspending
proppants which take the form of a proppant substrate particle
carrying a coating of a hydrogel-forming polymer. As further
described there, these proppants are formulated in such a way that
they rapidly swell when contacted with aqueous fracturing fluids to
form hydrogel coatings which are large enough to significantly
increase the buoyancy of these proppants during their transport
downhole yet durable enough to remain largely intact until they
reach their ultimate use locations. The disclosures of all of these
earlier applications are incorporated herein by reference in their
entireties.
[0003] It is well known that calcium and other cations can
substantially retard the ability of anionic hydrogel-forming
polymers to swell. This problem can be particularly troublesome
when self-suspending proppants made with such polymers are used,
because the waters to which the proppants are exposed, including
both the source water from which the associated fracturing fluid is
made up as well as the geological formation water which the
proppants encounter downhole, can often contain significant
quantities of these ions.
[0004] This problem, i.e., the tendency of calcium and other
cations to retard the ability of anionic hydrogel-forming polymers
to swell, can begin to occur when the hardness of the water
encountered by the polymer reaches levels as low as 300 ppm. In the
context of this document, the "hardness" of a water sample means
the sum of the concentrations of all divalent cations in the sample
in terms of an equivalent weight of calcium carbonate. For example,
a hardness of 1,000 ppm means that the total concentration of
divalent cations in the sample is the same as the concentration of
calcium cations that would be produced by 1,000 ppm by weight of
CaCO.sub.3 dissolved in pure water.
[0005] In many places in the United States especially where
hydraulic fracturing may be practiced, municipal waters (i.e., the
potable water produced by local municipalities) can have hardness
levels of 300 ppm or more, while naturally-occurring ground waters
can have hardness levels of 1,000 ppm or more. Meanwhile, sea water
has a hardness of approximately 6,400 ppm, while the geological
formation waters encountered downhole in many locations where
hydraulic fracturing occurs can have hardness levels even as high
as 40,000 ppm or even 80,000 ppm. That being the case, the
performance advantages of self-suspending proppants made with
anionic hydrogel-forming polymers can be adversely affected as the
hardness of the water to which the proppant is exposed
increases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIGS. 1-5 are photographs showing the settled bed heights of
some of the modified proppants described in Examples 3-5 of this
disclosure.
SUMMARY
[0007] We have now found that especially desirable salt-tolerant
self-suspending proppants can be made by forming the
water-swellable coating of the proppant from the combination of two
or more of an anionic hydrogel polymer, a cationic hydrogel polymer
and a nonionic hydrogel polymer.
[0008] Thus, this invention provides a self-suspending proppant
comprising a proppant substrate particle and a water-swellable
composite coating on the proppant substrate particle, wherein the
water-swellable composite coating comprises the combination of two
or more of an anionic hydrogel polymer, a cationic hydrogel polymer
and a nonionic hydrogel polymer.
[0009] In addition, this invention also provides an aqueous
fracturing fluid comprising an aqueous carrier liquid containing
the above self-suspending proppant.
[0010] In addition, this invention further provides a method for
fracturing a geological formation comprising pumping this
fracturing fluid into the formation.
DETAILED DESCRIPTION
Proppant Substrate Particle
[0011] As indicated above, the inventive self-suspending proppants
take the form of a proppant substrate particle carrying a
water-swellable composite coating. The substrate particle is
referred to herein as a particulate solid, substrate, proppant,
particulate material, proppant substrate particle, and particulate,
for example. These terms, in the context of referring to the
substrate, are intended to be interchangeable.
[0012] For this purpose, any particulate solid which has previously
been used or may be used in the future as a proppant in connection
with the recovery of oil, natural gas and/or natural gas liquids
from geological formations can be used. In this regard, see our
earlier filed application mentioned above and International Patent
Application No.: PCT/US13/32435, filed Mar. 15, 2013, entitled
Self-Suspending Proppants for Hydraulic Fracturing by Mahoney et
al., incorporated herein by reference, which identify many
different particulate materials which can be used for this purpose.
These materials can have densities as low as .about.1.2 g/cc and as
high as .about.5 g/cc and even higher, although the densities of
the vast majority will range between .about.1.8 g/cc and .about.5
g/cc, such as for example .about.2.3 to .about.3.5 g/cc, .about.3.6
to .about.4.6 g/cc, and .about.4.7 g/cc and more.
[0013] Specific examples include graded sand, bauxite, ceramic
materials, glass materials, polymeric materials, resinous
materials, rubber materials, nutshells that have been chipped,
ground, pulverized or crushed to a suitable size (e.g., walnut,
pecan, coconut, almond, ivory nut, brazil nut, and the like), seed
shells or fruit pits that have been chipped, ground, pulverized or
crushed to a suitable size (e.g., plum, olive, peach, cherry,
apricot, etc.), chipped, ground, pulverized or crushed materials
from other plants such as corn cobs, composites formed from a
binder and a filler material such as solid glass, glass
microspheres, fly ash, silica, alumina, fumed carbon, carbon black,
graphite, mica, boron, zirconia, talc, kaolin, titanium dioxide,
calcium silicate, and the like, as well as combinations of these
different materials. Especially interesting are intermediate
density ceramics (densities .about.3.1-3.5 g/cc), normal frac sand,
or frac sand, (density .about.2.65 g/cc), bauxite and high density
ceramics (density .about.3.5-5 g/cc), just to name a few.
Preferably, the material, or substrate, possesses sufficient
compression strength to withstand the pressure within the
geological formation, such as the compression strength of frac
sand.
[0014] In addition to these materials, resin coated varieties of
these materials can also be used. Specific examples include resin
coated sand, including sands coated with curable resins as well as
sands coated with precured resins. Other specific examples include
resin coated ceramic materials (light weight, intermediate density
and high density ceramics), including ceramics coated with curable
resins as well as ceramic coated with precured resins. In these
instances, the water-swellable coating of the inventive
self-suspending proppant will be understood to be "associated with"
the proppant substrate particle of this product rather than being
"on" this substrate particle. In other embodiments, the
water-swellable coating will provide a second coating, or outer
layer over, the resin.
[0015] As used herein, the term "particulate" includes, for
example, spherical materials, elongate materials, polygonal
materials, fibrous materials, irregular materials, and any mixture
thereof.
[0016] All of these substrates or particulate materials, as well as
any other particulate material which is used as a proppant in the
future, can be used to make the inventive self-suspending
proppants.
Water-Swellable Composite Coating
[0017] As indicated above, the inventive self-suspending proppants
are made in such a way that: [0018] (1) optionally and preferably,
they are free-flowing when dry, [0019] (2) they rapidly swell when
contacted with their aqueous fracturing fluids, [0020] (3) they
form hydrogel coatings which are large enough to significantly
increase their buoyancy during transport downhole, thereby making
these proppants self-suspending during this period, [0021] (4)
optionally and preferably, these hydrogel coatings are durable
enough to maintain the self-suspending character of these proppants
until they reach their final destination downhole, and [0022] (5)
these hydrogel coatings are especially resistant to the adverse
effects calcium and other cations can have on the swelling
properties of these coatings.
[0023] In accordance with this invention, this is accomplished by
forming the water-swellable composite coating of the inventive
self-suspending proppant from the combination of two or more of an
anionic hydrogel polymer, a cationic hydrogel polymer and a
nonionic hydrogel polymer.
[0024] The polymers can be added to the substrate independently, as
a mixture, simultaneously or sequentially.
[0025] For example, in some embodiments, these hydrogel polymers
can be combined with one another before they are added to the
proppant substrate particles. In these embodiments, the
water-swellable composite coating can be regarded as being formed
from a mixture of these hydrogel polymers. Depending on how much
these polymers are mixed before being added to the proppant
substrate particles, the distribution of these polymers in the
coating that is formed can be either homogeneous or
non-homogeneous. For example, the polymers can be added to a single
aqueous solution, optionally emulsified in an inverse emulsion, and
then added to the substrate for coating. Alternatively, a plurality
of inverse emulsions each comprising at least one water-swellable
polymer can be mixed and then added to the substrate.
[0026] In other embodiments, these hydrogel polymers (e.g., in an
aqueous solution, suspension or emulsion) can be separately
combined with the proppant substrate particles at the same time.
That is to say, they can be supplied to the manufacturing equipment
in which the water-swellable composite coating is formed from
separate sources but at the same time. In these embodiments, the
water-swellable composite coating can exhibit either a homogeneous
or non-homogeneous distribution of these hydrogel polymers
depending, for example, on the extent they mix as they deposit on
the proppant substrate particles.
[0027] In still other embodiments, these hydrogel polymers can be
separately combined with the proppant substrate particles at
different times, or sequentially. In these situations, the proppant
substrate particles will be at least partially coated with the
first-applied hydrogel polymer to form an undercoating, after which
an overcoating, or outer layer, formed from the second-applied
hydrogel polymer would be formed on this undercoating.
[0028] This overcoating approach can be carried out in two or more
different ways. In one way, formation of the overcoating is not
started until formation of the undercoating has been completed.
This can be done, for example, by drying the undercoating before
starting to form the overcoating or at least by allowing
essentially all of the hydrogel polymer forming the undercoating to
deposit on the proppant substrate particles before adding the
hydrogel polymer forming the overcoating. In this instance, the
water-swellable composite coating can, or may, be regarded as
comprising two or more distinct coating layers, an undercoating
made from the first-applied hydrogel polymer and an overcoating
made from the second-applied hydrogel polymer.
[0029] Another way this overcoating approach can be done is by
starting to form the overcoating before formation of the
undercoating is complete. In this instance, the water-swellable
composite coating will, or may, not be composed of two or more
distinct coating layers. Rather, it will, or may, be composed of a
mixture of the first-applied and second-applied hydrogel polymers
distributed in the composite coating in a non-uniform way, in
particular, with the concentration of the first-applied hydrogel
polymer decreasing and the concentration of the second-applied
hydrogel polymer increasing as the distance away from the surface
of the proppant substrate particle increases. In this embodiment,
one or more of the polymers may be present in the water-swellable
coating in a gradient.
[0030] Instead of forming the water-swellable composite coating of
the inventive self-suspending proppant from two coating layers in
the manner discussed above, it can also be formed from three or
more coating layers, e.g., one being made from the anionic hydrogel
polymer, another being made from the cationic hydrogel polymer and
the third being made from the nonionic hydrogel polymer. If so,
these three different hydrogel polymer layers can be arranged in
any order with respect to one another. The layers may be distinct
layers or may form one or more gradients, as discussed above. In a
preferred embodiment, the outermost coating layer will be formed
from the cationic or nonionic hydrogel polymer, and especially the
cationic hydrogel polymer. Preferably, the anionic hydrogel polymer
will be localized in an inner layer, or concentrated or localized
on the surface of the substrate.
[0031] Also, in the same way as discussed above in connection with
two-layer water-soluble composite coatings when three-layer
water-soluble composite coatings are made, formation of the
intermediate and outer coating layers can begin before formation of
the preceding layer is complete so that the water-soluble composite
coating obtained, rather than being formed from three distinct
coating layers, is formed from a mixture of the first-applied,
second-applied and third-applied hydrogel polymers distributed in
the composite coating in a non-uniform way, in particular, with the
concentration of the previously-applied hydrogel polymer decreasing
and the concentration of the subsequently-applied hydrogel polymer
increasing as the distance away from the surface of the proppant
substrate particle increases.
[0032] Another way water-swellable composite coatings of the
inventive self-suspending proppants can be made using all three of
an anionic hydrogel polymer, a cationic hydrogel polymer and a
nonionic hydrogel polymer is to make a two-layer water-soluble
composite coating with one or both of these layers being composed a
homogeneous or non-homogeneous mixture of two of these hydrogel
polymers but not the third. For example, the water-swellable
coating can be composed of an undercoating comprising an anionic
hydrogel polymer such as an anionic polyacrylamide and an
overcoating comprising the combination of a cationic hydrogel
polymer and a nonionic hydrogel polymer.
[0033] Yet another way water-swellable composite coatings of the
inventive self-suspending proppants can be made from all three of
an anionic hydrogel polymer, a cationic hydrogel polymer and a
nonionic hydrogel polymer is to make this water-swellable composite
coating from a homogeneous or non-homogeneous mixture of all three
of these hydrogel polymers.
[0034] Generally, the water-soluble composite coating of the
inventive self-suspending proppants will be composed of either one,
two or three coating layers, as discussed above, it being
understood that when two or three coating layers are involved these
different coating layers can either be distinct or non-distinct in
the sense that the hydrogel polymers forming these different layers
are distributed in the water-swellable composite coating in a
non-uniform way.
[0035] However, it is also possible in accordance with this
invention that additional coating layers can be included in the
inventive self-suspending proppants, with these additional coating
layers being located underneath, on top of, or in between coating
layers forming this water-soluble composite coating. In most
instances, however, the inventive self-suspending proppant will be
structured so that the outermost coating layer of the proppant
comprises a cationic hydrogel polymer, a non-ionic hydrogel
polymer, or a mixture of both. Inventive self-suspending proppant
in which the outermost coating layer of the proppant comprises a
cationic starch, a pre-crosslinked cold water-swellable starch, or
a mixture of both, are especially interesting.
Anionic Hydrogel Polymer
[0036] The anionic hydrogel polymers which can be used to form the
water-swellable composite coating of the inventive self-suspending
proppant include any polymer which is capable of forming a hydrogel
when exposed to water and which, in addition, exhibits anionic
functionality can be used. Mixtures of these polymers can also be
used. Basically, these polymers take the form of a polymer or
copolymer which is capable of forming a hydrogel and which has been
made from a monomer or comonomer which exhibits anionic
functionality, or which is treated after it is made to impart
anionic functionality, or both.
[0037] Examples of synthetic polymers, which are capable of forming
hydrogels, include polymers and copolymers of acrylamide, polymers
and copolymers of acrylic acid and its salts, polyvinylalcohols,
polyurethanes, polyethylene glycols, polypropylene glycols, betaine
esters, amino acid-based poly(ester amides) (AA-PEAs) and
polysiloxanes.
[0038] Examples of naturally occurring polymers, which are capable
of forming hydrogels, are various polysaccharides such as starches
including modified starches such as acid-modified starches,
alkylated starches, oxidized starches, acetylated starches,
dextrans, dextrins, and so forth. Natural gum polymers such as guar
gum, carboxymethyl guar and carboxymethyl hydroxypropyl guar gum
can also be used, as can cellulose based polymers such as cellulose
and cellulose derivatives including alkyl cellulose ethers such as
methyl cellulose, ethyl cellulose and/or propyl cellulose, hydroxy
cellulose ethers such as hydroxy methyl cellulose, hydroxy ethyl
cellulose, carboxymethyl cellulose, and/or hydroxy propyl
cellulose, cellulose esters such as cellulose acetate, cellulose
triacetate, cellulose propionate and/or cellulose butyrate,
cellulose nitrate and cellulose sulfate. Also useful are chitosan,
glycogen and biopolymers such as proteins, protein hydrolysates,
and the like. Mixtures of these materials can also be used.
[0039] Examples of moieties which can be included in such polymers
for exhibiting anionic functionality include carboxyl groups, metal
carboxylate groups especially those in which the metal is an
alkaline or alkaline earth metal, sulfonates and phosphates.
[0040] Any polymer which is capable of forming a hydrogel when
exposed to water and which, in addition, exhibits anionic
functionality can be used for carrying out this invention. They are
well known and described, for example, in commonly assigned U.S.
Pat. No. 9,297,244 (7-US) and U.S. Pat. No. 9,315,721 (4-US), the
disclosures of which are incorporated herein by reference.
Generally, these polymers will have weight average molecular
weights on the order of 1,000,000 to 60,000,000 Daltons, more
typically, 5,000,000 to 40,000,000 Daltons or even 10,000,000 to
30,000,000 Daltons, and charge densities (or degrees of hydrolysis)
of 5 to 90 mole %, more typically, 10 to 60 mole %, 15 to 50 mole
%, or even 20 to 40 mole %. In this context, "charge density" will
be understood to mean the net negative charge imparted by the
anionic group expressed as mole %, i.e., the mole % of monomers in
the polymer which exhibit anionic functionality.
[0041] Anionic hydrogel polymers of special interest are the
anionic polyacrylamides. An example of such an anionic
polyacrylamide is given by the following formula:
##STR00001##
wherein [0042] m is the molar fraction of acrylamide in the
copolymer and ranges from 0.05 to 0.9, more typically 0.2 to 0.6,
0.15 to 0.50, or even 0.2 to 0.4, [0043] n is the molar fraction of
anionic comonomer in the copolymer, and [0044]
0.9.ltoreq.(m+n).ltoreq.1. Generally, (m+n) will be at least 0.95,
at least 0.98, or even 1. [0045] R may also be other monovalent
substituents, especially alkali metal.
[0046] Polymethacrylamides are also contemplated. In an especially
interesting embodiment of this invention, the anionic hydrogel
polymer is a hydrolyzed polyacrylamide. Generally speaking, there
are two primary ways of making the above anionic polyacrylamide
commercially, (1) copolymerizing acrylamide with a comonomer
exhibiting anionic functionality such as acrylic acid or sodium
acrylate and (2) hydrolyzing a polyacrylamide homopolymer by
contact with a strong acid or base. In accordance with this
invention, it has been found that hydrolyzed polyacrylamides, i.e.,
anionic polyacrylamides made by hydrolyzing a polyacrylamide
homopolymer, especially those made by hydrolyzing with a strong
base, produce self-suspending proppants with especially good hard
water tolerance.
[0047] Although not wishing to be bound to any theory, it is
believed that hydrolyzed anionic polyacrylamides exhibit this
enhanced hard water tolerance because the pendant carboxylic groups
which are produced by hydrolysis are distributed in the polymer
chain with a greater degree of non-uniformity (i.e., more randomly)
as compared with anionic polyacrylamides made by other techniques.
As a result, the ability of the polymer to bind the divalent
calcium or magnesium cations in hard water is less, because the
number of instances in which two pendant carboxylic groups are
directly adjacent one another in the polymer chain is less.
[0048] It is believed random distribution of pendant carboxylic
groups does not occur to the same extent when other techniques are
used to make the anionic polyacrylamides. As a result, the polymer
chain of a copolymerized acrylate and acrylamide monomers has more
pairs and triads of directly adjacent pendant carboxylic groups
which are capable of taking up and binding the divalent calcium and
magnesium cations found in hard water. Therefore, when such a
polymer is exposed to hard water, more of its pendant carboxylic
groups are taken up by binding calcium and magnesium ions which, in
turn, reduces the number ofthese pendant carboxylic groups which
are available for taking up and "binding" water molecules. Since it
is this taking up and binding of water molecules which is
responsible for polymer swelling, the net effect of this uniform
distribution of pendant carboxylic groups is that the ability of
these polymers to swell when exposed to hard water is less. See,
Truong et al., Effect of the Carboxylate Group Distribution on the
Potentiometric Titration of Acrylamide-Acrylic Acid Copolymers,
Polymer Bulletin 24, 101-106 .COPYRGT. Springer-Verlag 1990.
[0049] As in the case of the other anionic polyacrylamides
described above, it is desirable that the hydrolyzed anionic
polyacrylamides described here also exhibit a charge density (or
degree of hydrolysis) of 5 to 90 mole %, more typically, 10 to 60
mole %, 15 to 50 mole %, or even 20 to 40 mole %.
[0050] Preferred anionic polyacrylamides include polyacrylamide
inverse emulsions, particularly high molecular weight
polyacrylamide inverse emulsions. Preferred polyacrylamides include
the FLOPAM series of polyacrylamides, particularly, FLOPAM EM533,
from SNF. Such polyacrylamides form interpenetrating networks when
contacted with the substrate, optionally crosslinked, dewatered and
dried, creating a shear-stable cage surrounding the substrate
particles.
Cationic Hydrogel Polymer Coating
[0051] The cationic hydrogel polymers which can be used to form the
water-swellable composite coating of the inventive self-suspending
proppant include any polymer which is capable of forming a hydrogel
when exposed to water and which, in addition, exhibits cationic
functionality. Mixtures of these polymers can also be used. Like
the anionic hydrogel polymers described above, these polymers also
take the form of a polymer or copolymer which is capable of forming
a hydrogel. However, in this instance, these polymers have been
made from a monomer or comonomer which exhibits cationic
functionality, or have been treated after being made to impart
cationic functionality, or both.
[0052] These polymers can be made from the same hydrogel polymers
from which the anionic hydrogel polymers described above are
made.
[0053] In order to impart cationic functionality to these polymers,
any known cationic reagent can be used, examples of which include
amino groups, imino groups, sulfonium ions, phosphonium ions,
ammonium ions and mixtures thereof. Generally, these polymers will
have weight average molecular weights on the order of 1,000,000 to
60,000,000 Daltons, more typically, 5,000,000 to 40,000,000 Daltons
or even 10,000,000 to 30,000,000 Daltons, and charge densities 5 to
90 mole %, more typically, 10 to 60 mole %, 15 to 50 mole %, or
even 20 to 40 mole %.
[0054] Cationic hydrogel polymers of special interest are the
cationic starches, such as starches that are at least partially
gelatinized in form. Examples of suitable starches which can be
used for this purpose include naturally-occurring starches,
acid-modified starches, pre-crosslinked starches, alkylated
starches, oxidized starches, acetylated starches, hydroxypropylated
starches, monophosphorylated starches, octenylscuccinylated
starches and so forth.
[0055] Starches can be anionic, cationic and amphoteric, depending
primarily on the nature of the substituents present at the 2, 3, 5
and 6 positions of the monosaccharide units forming the starch
molecule. In accordance with this embodiment of the invention,
cationic starches are used to make the cationic hydrogel coatings
of the inventive self-suspending proppants, especially those
cationic starches which have a degree of substitution (i.e.,
cationic degree of substitution) of 0.017 to 0.55 or higher. Those
cationic starches having a degree of substitution of 0.030 to 0.55,
0.15 to 0.45 or even 0.2 to 0.4 are even more interesting. Of these
cationic starches, those having from about 1 to 50 wt. %, more
typically about 5 to 30 wt. % or even about 10 to 25 wt. % of
amylase (linear polymer) units and about 50 to 99 wt. %, more
typically about 70 to 95 wt. % or even about 75 to 90 wt. % of
amylopectin (branched polymer) are especially interesting. Also
especially interesting are those cationic starches whose cationic
functionality is based on quaternary ammonium groups.
[0056] Those of the above cationic starches having both a high
degree of substitution as represented by a degree of substitution
of at least about 0.04, preferably at least about 0.1, and a low
amylase content, i.e., 10 wt. % or lower, are especially
interesting.
[0057] Cationic starches which are useful in this invention also
typically have molecular weights of about 1 to 8 million Daltons,
more typically about 2 to 6 million Daltons, although higher and
lower molecular weights are still possible.
[0058] A wide variety of different commercially available cationic
starches can be used for the purposes of this invention. Examples
include the ALTRA-CHARGE line of cationic starches available from
Cargill, Incorporated of Wayzata, Minn., the STA-LOK and INTERBOND
line of cationic starches available from Tate & Lyle of
Decatur, Ill., and the CHARGEMASTER line of cationic starches
available from Grain Processing Corporation of Muscatine, Iowa They
are available in different forms including powders, aqueous pastes,
aqueous slurries, aqueous dispersions and aqueous solutions, all of
which can be used to make the self-suspending proppants of this
invention.
[0059] Specific examples include CHARGEMASTER R31F, R32F, R33F,
R43F, R25F, R67F, R467, R62F, R63F and R65F, INTERBOND.RTM. C,
STA-LOK.RTM. 120, 156, 160, 180, 182, 190, 300, 310, 330, 356 and
376, and ALTRA CHARGE.TM. 240 and 340 and others. Specific examples
of cationic starches in paste or slurry form that can be used for
this purpose include CHARGEMASTER L435, L340 and L360.
[0060] In addition to purchasing commercially available cationic
starches, these materials can also be made in-house if desired. For
example, a starch can be made cationic by reacting it with any
known cationic reagent, examples of which include reagents having
amino groups, imino groups, sulfonium ions, phosphonium ions, or
ammonium ions and mixtures thereof. The cationization reaction may
be carried out in any conventional manner such as reacting the
starch with the cationic reagent in an aqueous slurry, usually in
the presence of an activating agent such as a base like sodium
hydroxide. Another process that may be used is a semi-dry process
in which the starch is reacted with the cationic reagent in the
presence of an activating agent such as a base like sodium
hydroxide in a limited amount of water.
[0061] Especially interesting cationic reagents that can be used
for this purpose are those based on quaternary ammonium compounds
in either epoxy or chlorohydrin form. This is because the epoxy and
chlorohydrin functionalities of these compounds react quickly with
the pendant alcohol groups of the starch polymer while their
quaternary ammonium groups provide the cationic functionality to
the polymer. Specific examples include
(3-chloro-2-hydroxypropyl)trimethylammonium chloride and
2,3-epoxypropyltrimethylammonium chloride. Techniques for preparing
cationic starches are well known and described in numerous
references. See, for example, U.S. Pat. No. 4,554,021. See, also,
QUAB.RTM. Cationization of Polymer, Product literature of SKW Quab
Chemicals, Inc. of Saddle Brook, N.J., pp 1-11. Also, see, Moad,
Chemical Modification of Starch by Reactive Extrusion, Progress in
Polymer Science 36 (2011) 218-237. In addition, please also note
Properties of Modified Starches and their Use in the Surface
Treatment of Paper, Dissertation of Anna Jonhed, 2006:42, at
http://www.diva-portal.org/smash/get/diva2:6450/FULLTEXT01.pdf,
Karlstad University 2006. The disclosures of each of these
references are incorporated herein by reference in their
entireties.
[0062] As indicated above, the cationic starches which are used to
make the water-swellable composite coatings of the inventive
self-suspending proppant are at least partially gelatinized. Starch
molecules arrange themselves in plants in semi-crystalline
granules. Heating in water causes water molecules to diffuse
through these granules, causing them to become progressively
hydrated and swell. In addition, their amylase content depletes
through leaching out by the water. When further heated, these
granules "melt" or "destructure" in the sense that their
semi-crystalline structure is lost, which can be detected by a
variety of different means including X-ray scattering,
light-scattering, optical microscopy (birefringence using crossed
polarizers), thermomechanical analysis and NMR spectroscopy, for
example. This "melting-destructuration" effect is known as
gelatinization. See, Kalia & Avernus, Biopolymers: Biomedical
and Environmental Applications, p. 89, .COPYRGT. 2011 by Scrivener
Publishing LLC, Co-published by John Wiley & Sons, Hoboken,
N.J.
[0063] Incidentally, for convenience, in this disclosure we use the
term "gelatinized" and "gelatinous" in connection with starches to
refer both to starches which are only partially gelatinized as well
as to starches which are fully gelatinized in the sense of being
incapable of taking up any more water of gelatinization. In
addition, we use the term "dried" in connection with these starches
to refer to starches which have undergone this gelatinization
procedure and then are subsequently dried, whether gelatinization
and drying occur in-house or have already occurred at the
manufacturer before purchase.
[0064] A convenient way of insuring that the desired degree of
starch gelatinization is achieved when using starches that have not
been previously gelatinized is to control the water/cationic starch
weight ratio of the water/starch coating composition which is used
to make the inventive self-suspending proppants. Generally, this
ratio can range from about 0.05:1 to 15:1, although water/starch
weight ratios of 0.5:1 to 10:1, 0.75:1 to 7.5:1, 1:1 to 5:1, 1.25:1
to 4:1, and even 1.5:1 to 3:1, are contemplated. And for this
calculation, it will be understood that all of the water present in
this coating composition will be taken into account including the
moisture/water content of the raw material cationic starch used,
any water that might be present from applying the other
water-swellable coating layer, any make-up water that might be
added, and the water content of any ingredient that might be used
such as crosslinking agents and the like.
[0065] Starch gelatinization generally requires that the
starch-water combination have a slightly alkaline pH such as
.gtoreq.7.5, .gtoreq.8, .gtoreq.9, and even .gtoreq.10 as well that
the starch-water combination be heated to above a characteristic
temperature, known as the gelatinization temperature. See, the
above-noted Kalia publication. So, in carrying out this embodiment
of the invention, heating of the cationic starch under suitable
conditions to achieve at least partial starch gelatinization may be
necessary, if the raw material starch that is being used has not
been previously gelatinized.
[0066] In addition to using the above cationic starches, copolymers
or block polymers of these cationic or even neutral starches with
other vinyl comonomers or polymers can also be used. Examples
include acrylamides, acrylates, methacrylates,
2-acrylamido-2-methylpropanesulfonic acid (AMPS), vinyl acetate,
vinyl alcohol and so forth. Desirably, these cationic starch
copolymers have the same degree of substitution mentioned above.
That is, the degree of substitution provided by the cationic
functionality of these copolymers is the same as mentioned
above.
[0067] Techniques for forming water-swellable coating layers made
from at least partially gelatinized cationic starches on proppants
substrate particles are more fully described in the above-mentioned
commonly-assigned application U.S. Ser. No. 62/337,547
(17922/05168), the disclosure of which is incorporated herein by
reference in its entirety.
[0068] Another type of cationic hydrogel-forming polymer of special
interest in connection with making the water-swellable composite
coating of the inventive self-suspending proppant are the cationic
polyacrylamides. These polymers are copolymers of acrylamide and
one or more additional comonomers capable of introducing cationic
functionality into the polymer. They also may be chemically
modified polyacrylamides made by introducing one or more cationic
moieties. This cationic functionality can be based on a variety of
different pendant cationic groups including quaternary ammonium
compounds, phosphonium salts and sulphonium salts. Such cationic
polyacrylamides typically have weight average molecular weights on
the order of 100,000 to 60,000,000 Daltons, more typically, 500,000
to 40,000,000 Daltons or even 10,000,000 to 30,000,000 Daltons, and
charge densities of 5 to 85 mole %, more typically, 10 to 80 mole %
or even 15 to 70 mole %.
[0069] An example of such a cationic polyacrylamide is given by the
following formula:
##STR00002##
wherein [0070] m is the molar fraction of acrylamide or
methacrylamide in the copolymer, [0071] n is the molar fraction of
cationic comonomer in the copolymer, [0072] m and n are each
independently within the range of from 0 to 1, [0073]
(m+n).ltoreq.1, [0074] R.sub.1 is hydrogen or methyl, [0075]
R.sub.2 is hydrogen or methyl, [0076] A.sub.1 is --O-- or --NH--,
[0077] R.sub.3 is alkylene having from 1 to 3 carbon atoms or
hydroxypropylene, [0078] R.sub.4, R.sub.5 and R.sub.6 are each
independently methyl or ethyl or other alkyl having from 3 to 12
carbon atoms, and [0079] X is an anionic counter ion, such as, for
example, chloride, bromide, methyl sulfate, ethyl sulfate or the
like. Note that, when A.sub.1 is --NH--, it can be considered as
chemically modified polyacrylamide rather than a copolymer
theoretically.
[0080] In a particular polyacrylamide of this type, the molar ratio
of acrylamide (m) to cationic monomer (n) is in the range of 0:1 to
0.95:0.05, while the sum of the molar ratios of m and n is 1. The
cationic polyacrylamides of the above formula can be random or
block copolymers.
Nonionic Hydrogel Polymer Coating
[0081] The nonionic hydrogel polymers which can be used to form the
water-swellable composite coating of the inventive self-suspending
proppant include any polymer which is capable of forming a hydrogel
when exposed to water and which, in addition, exhibits little or no
anionic or cationic functionality can be used. Mixtures of these
polymers can also be used.
[0082] These polymers can be made from the same hydrogel polymers
from which the anionic and cationic hydrogel polymers described
above are made.
[0083] Of these nonionic hydrogel polymers, those which are cold
water-swellable are of interest. In this context, "cold
water-swellable" means that the polymer will form a relatively
homogeneous hydrogel mass in room temperature water with gentle
mixing. These hydrogel polymers are interesting because they can be
directly used in powder form, as received from the manufacturer.
That is to say, they can be added to the other ingredients forming
the water-swellable composite coating without dilution in a carrier
liquid first. On the other hand, they can also be dissolved or
dispersed in a suitable carrier liquid such as water, isopropyl
alcohol or various organic solvents such as mineral oils, various
alkanes such as n-hexane, various commercially available
isoparaffinic solvents, and the like before being added to these
other ingredients.
[0084] In addition to cold water-swellable nonionic hydrogel
polymers, pre-crosslinked nonionic hydrogel polymers are also
interesting. Nonionic hydrogel polymers which are both cold
water-swellable and pre-crosslinked are even more interesting.
[0085] Especially interesting nonionic hydrogel polymers are the
pre-crosslinked, cold water-swellable starches. Examples include
hydroxypropylated di-starch phosphate (HDP), which is commercially
available from Cargill as HiForm 12750. Other examples include
PolarTex Inst 12640 and StabiTex Inst 12620 also available from
Cargill.
[0086] Additionally or alternatively, the anionic polymer, cationic
polymer and/or nonionic polymer can be covalently linked, as can be
found in a block polymer. Additionally and/or alternatively, the
anionic, cationic and/or nonionic polymers can be water-soluble and
subsequently crosslinked to render them water-swellable. Thus,
aqueous polymer solutions and/or inverse emulsions are added to the
substrate and then crosslinked, as discussed in more detail
below.
[0087] In an especially interesting embodiment of this invention,
the inventive self-suspending proppants are made by a continuous
process in which an anionic polymer in a suitable carrier liquid,
such as water or an inverse emulsion, is added to the proppant
substrate particles first, following which the remaining polymers
are added while the anionic hydrogel polymer is still wet with its
carrier liquid. Mixing is then continued until the hydrogel
polymers have deposited onto the proppant substrate particles,
after which the coated product so formed is dried.
[0088] In this continuous process, these remaining hydrogel
polymers can also be supplied in their own carrier liquids, such as
water or an inverse emulsion, if desired. However, in those
embodiments in which the remaining hydrogel polymers include a cold
water-swellable nonionic hydrogel polymer, this hydrogel polymer
can be added to the previously formed mixture of anionic hydrogel
polymer and proppant substrate particles in powder form without
dilution in its own carrier liquid, since it will readily disperse
in and gel when contacted with the aqueous carrier liquid of the
anionic hydrogel polymer. Pre-crosslinked cold water-swellable
neutral starches work especially well for this purpose.
Coating Amounts
[0089] The total amount of hydrogel polymers used to make the
water-swellable composite coating of inventive self-suspending
proppants depends among other things on the degree or extent to
which it is desired to increase the buoyancy of these proppants.
One way this enhanced buoyancy can be quantified is by comparing
the thickness of the water-swellable composite coating that is
formed after it has expanded through contact with an excess of
water with the average diameter of the proppant particle
substrate.
[0090] Another way this enhanced buoyancy can be quantified is by
determining the settled bed height of the inventive self-suspending
proppant when fully expanded with water with the settled bed height
of an equivalent amount of uncoated proppant substrate
particles.
[0091] Still another way this enhanced buoyancy can be quantified
is by comparing the density of the inventive self-suspending
proppant when fully expanded with water to the density of the
proppant substrate particle from which it is made. For example,
normal frac sand has a density (e.g., an apparent specific gravity)
of -2.65 g/cc, whereas a self-suspending proppant made in
accordance with this invention from this substrate particle can
have a density of about 1.5 g/cc when fully expanded, for example.
This means that the water-swellable composite coating of this
invention has been able to decrease the effective density of this
self-suspending proppant by about 1.15 g/cc or by about 40% or
more, approximating the density of the aqueous fracturing
fluid.
[0092] In carrying out this invention, the relative amounts of the
water-swellable composite coating and proppant substrate particles
used can vary widely, and essentially any amounts can be used. In
some embodiments, the amount used will be sufficient so that the
thickness of the water-swellable composite coating which is formed
when fully expanded with water is 10% to 1000% of the average
diameter of the proppant particle substrate. Water-swellable
composite coating thicknesses of 25% to 750%, 50% to 500% and 100%
to 300% of the average diameter of the proppant particle substrate
are contemplated.
[0093] Additionally or alternatively, the amount of water-swellable
composite coating used will be sufficient so that the Settled Bed
Height of the product, as can be determined in the manner discussed
more fully below, is at least 130%, more desirably, at least 150%,
at least 175%, at least 200%, at least 250%, at least 300%, at
least 350% and even at least 400% of the Settled Bed Height of an
equivalent amount of uncoated proppant substrate particles.
[0094] Additionally or alternatively, the amount of water-swellable
composite coating used will be sufficient so that a decrease in
density of at least about 0.25 g/cc, determined as described above,
is achieved. More typically, the decrease in density will be at
least about 0.50 g/cc, at least about 0.75 g/cc, at least about
1.00 g/cc, at least about 1.25 g/cc, or even at least about 1.50
g/cc. Additionally or alternatively, the density of the product is
at least about 25%, preferably at least about 30%, more preferably
at least about 40% less than the density of the substrate.
Additionally or alternatively, the density of the product, when
swelled in water is between about 0.75 and about 1.25 g/cc, such as
between about 0.9 and 1.15 g/cc.
[0095] Meanwhile, the maximum amount of water-swellable composite
coating used will generally be limited by practical considerations
in the sense that this amount is desirably not so much that no
practical advantage is realized in terms of the increase in
buoyancy provided by this coating. That is, as the density of the
swellable product, as a function of the coating thickness, will
substantially plateau. The coating thickness at which the product
density plateaus can be determined by routine experimentation.
Thus, the amount, or thickness, of the coating layer will
preferably have a thickness which is less than the amount at which
the density plateaus.
[0096] For example, in embodiments of this invention in which frac
sand (density .about.2.65 g/cc) is used as the proppant substrate
particle, the amount of water-swellable composite coating used on a
dry weight basis will generally be about 0.5 to 40 wt. %, more
typically 0.75 to 20 wt. %, 1 to 15 wt. %, about 1.3-12 wt. %, or
even 2-10 wt. % based on the weight of the frac sand used. In some
embodiments, for example where a greater degree of buoyancy is
desired, the amount of water-swellable composite coating used on a
dry weight basis will generally be about 3 to 40 wt. %, more
typically 3.3 to 20 wt. %, 3.5 to 15 wt. %, about 3.75-10 wt. %, or
even 4-8 wt. % based on the weight of the frac sand used. In other
embodiments, for example where less buoyancy is desired, the amount
of water-swellable composite coating used on a dry weight basis
will generally be about 0.5 to 20 wt. %, more typically about 1 to
15 wt. %, about 1.25-10 wt. %, about 1.5-7.5 wt. %, about 1.75-5
wt. % or even about 2-4 wt. % based on the weight of the frac sand
used.
[0097] When other proppant substrate particles are used, comparable
amounts of these hydrogel polymers can be used. For example, if an
intermediate density ceramic having a density of about 3.27 g/cc is
used, the amount of water-swellable composite coating used on a dry
weight basis can be about 1.23 (3.27/2.65) times the above amounts
on a dry weight basis if the same relative increase in buoyancy is
desired. If a greater amount of buoyancy is desired, more
water-swellable composite coating can be used, while if a less
amount of buoyancy is desired, less water-swellable composite
coating can be used, all of which can be easily determined by
routine experimentation.
[0098] In this regard, a particular advantage of this invention as
compared with our earlier invention in which a gelatinized cationic
starch is used as the water-swellable polymer, as described in the
above-noted U.S. Ser. No. 62/337,547 (Atty. Docket 17922/05168), is
that the amount of water-swellable composite coating that is needed
to achieve a given amount of buoyancy in aqueous liquids containing
high concentrations of calcium and other cations is considerably
less than that required to make the self-suspending proppants of
our earlier invention. For example, when comparing the inventive
self-suspending proppants with those of our earlier invention on an
equivalent basis, i.e., when made with the same sand particle
substrate and tested in the same calcium ion-rich aqueous test
liquid to achieve essentially the same increase in buoyancy, it
takes only about 4 to 5 wt. % of the water-swellable composite
coating of this invention as compared with the 7 to 9 wt. %
cationic hydrogel polymer of our earlier invention. This can
represent a significant savings in material costs.
[0099] More importantly, this difference also translates into a
substantial savings in production costs, because the total amount
of volatiles involved when producing the inventive self-suspending
proppants is less than involved when producing our earlier
self-suspending proppants. The hydrogel polymers used in this
invention and our earlier invention can preferably be supplied in
the form of polymer emulsions. To produce product proppants in dry
form, the carrier liquids in these emulsions need to be driven off,
for example, by evaporation through heating. Because the total
amount of hydrogel polymer is less when the inventive proppants are
made, the energy costs needed to achieve this evaporation is
correspondingly less.
[0100] For example, the total volatiles involved in making a
self-suspending proppant containing 9.7 wt. % cationic starch in
accordance with our earlier invention is about 355.6 lb/ton. In
contrast, the total volatiles involved in making a roughly
equivalent self-suspending proppant containing 4.0 wt. % of the
water-swellable composite coating of this invention is only about
114.5 lb/ton. Thus, the cost of producing this inventive proppant
would be considerably less, because less energy is needed to drive
off the volatiles involved in its production.
[0101] A still further advantage of this invention as it relates to
our earlier invention is temperature control during the proppant
gelatinization/drying process when a starch is used as one of the
hydrogel polymers. When the self-suspending proppants of our
earlier invention are made using large amounts of starch, starch
gelatinization involved controlled ramping of the temperatures from
gelatinization through drying. This controlled temperature ramping
is unnecessary when the inventive proppants are made, as it has
been found that sufficient starch gelatinization occurs when these
proppants are made as described, for example, in commonly assigned
U.S. Pat. No. 9,297,244 and U.S. Pat. No. 9,315,721, the
disclosures of which are incorporated herein by reference.
[0102] The relative amounts of the anionic, cationic and nonionic
hydrogel polymers in the water-swellable composite coating of the
inventive self-suspending proppant are not critical, and
essentially any amounts can be used, provided that this coating
comprises at least two of these different types of hydrogel
polymers. Typically, the cationic polymer is added to the coating
in at least an amount sufficient to bind ions in the fracturing
fluid. Typically, the anionic polymer is more effective at swelling
than the cationic polymer or nonionic polymer. Accordingly the
anionic polymer is added to the coating in an amount sufficient to
provide the degree of swelling and buoyancy discussed above.
Typically, the nonionic polymer, particularly a starch-based
polymer, is less expensive than the anionic polymer and is added to
the coating in an amount to reduce the cost of the coating and
absorb any ions not bound by the cationic polymer.
[0103] When this coating comprises the combination of only two of
these hydrogel polymers, such combinations will generally include
at least 10 wt. % of each of these hydrogel polymers, more
typically at least 20 wt. %, at least 30 wt. % or even at least 40
wt. % of each of these hydrogel polymers. On the other hand, when
this coating comprises the combination of all three of these
hydrogel polymers, such combinations will generally include at
least 1 wt. % of each of these hydrogel polymers, more typically at
least 3 wt. %, at least 5 wt. %, at least 10 wt. %, at least 15
wt., at least 20 wt. %, or even at least 30 wt. % of each of these
hydrogel polymers. In both cases, it will be understood that these
proportions apply whether each type of hydrogel polymer is composed
of only a single hydrogel polymer or mixtures of two of more
hydrogel polymers of the same type.
[0104] Alternatively or additionally, the amount of the anionic
polymer, when present, to total polymer will be at least about 3,
such as at least about 5 wt %, preferably at least 10 wt %, on a
dry weight basis. Preferably, the amount of the anionic polymer to
total polymer will be less than about 50 wt %, preferably less than
30 wt %, on a dry weight basis. Alternatively or additionally, the
amount of the cationic polymer, when present, to total polymer will
be at least about 15 wt %, 25 wt %, 30 wt %, 35% wt %, 40% wt %,
45% wt %, 50% wt %, 55% wt %, 60% wt %, 65% wt %, 70% wt %, 75% wt
%, or 80% wt %, each on a dry weight basis. Preferably, the amount
of the nonionic polymer, when present, to total polymer will be at
least about 15, such as at least about 25 wt %, preferably at least
30 wt %, more preferably at least about 50% on a dry weight
basis.
[0105] In an embodiment, the coating consists of a covalent
crosslinker, anionic polymer, cationic polymer and optional
nonionic polymer. In an embodiment, the cationic and anionic
polymers are each polyacrylamides present in a ratio of about 4:1
by weight.
[0106] In certain embodiments of the invention, as described above,
the water-swellable composite coating of the inventive
self-suspending proppant will be composed of two coating layers, a
first-applied coating layer and a second-applied coating layer,
with one of these coating layers comprising an anionic hydrogel
polymer and the other of these coating layers comprising a cationic
hydrogel polymer, a nonionic hydrogel polymer or the combination of
both. Generally, the first-applied coating layer will be formed
from the anionic hydrogel polymer, but it is also possible that
this first-applied coating layer can be formed from the cationic
hydrogel polymer, as well. In both cases, the relative amounts of
the first-applied coating layer and the second-applied coating
layer can vary widely and essentially any relative amount can be
used. Generally, the amounts used will be such that the ratio of
the first-applied coating layer to the second-applied coating
layer, on a dry weight basis, is 1:6 to 5:1, more typically, 1:4 to
3:1, 1:3 to 2:1, or even 1:2 to 1:1.
Chemical Modification for Enhancing Coating Adhesion
[0107] In order to improve the durability of the water-swellable
composite coating of the inventive self-suspending proppant once
swollen with its aqueous hydraulic fracturing fluid, one or more of
its elements, including all of its elements, can be chemically
treated by one or more adhesion-promoting approaches. In this
context, the "elements" of the inventive self-suspending proppants
will be understood to include its proppant substrate particle and
its water-swellable composite coating, as well as each of the
individual components and/or ingredients forming this coating.
[0108] In accordance with one such approach, one or more of the
hydrogel polymers forming this coating can be crosslinked. For this
purpose any di- or polyfunctional crosslinking agent having two or
more functional groups capable of reacting with the anionic and/or
cationic hydrogel polymer can be used. Examples include organic
compounds containing and/or capable of generating at least two of
the following functional groups: epoxy, carboxy, aldehyde,
isocyanate, amide, vinyl, and allyl. In some instances, especially
when the anionic hydrogel polymer is being crosslinked,
polyfunctional inorganic compounds such as borates, zirconates,
silicas and their derivatives can also be used as can guar and its
derivatives.
[0109] Specific examples of polyfunctional crosslinking agents that
can be used in this invention include epichlorohydrin,
polycarboxylic acids, carboxylic acid anhydrides such as maleic
anhydride, carbodiimide, formaldehyde, glyoxal, glutaraldehyde,
various diglycidyl ethers such as polypropylene glycol diglycidyl
ether and ethylene glycol diglycidyl ether, other di-or
polyfunctional epoxy compounds, phosphorous oxychloride, sodium
trimetaphosphate and various di-or polyfunctional isocyanates such
as toluene diisocyanate, methylene diphenyl diisocyanate and
polymers thereof, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide,
methylene bis acrylamide, naphthalenediisocyanate,
xylene-diisocyanate, tetramethylene diisocyanate, hexamethylene
diisocyanate, trimethylene diisocyanate, trimethyl hexamethylene
diisocyanate, cyclohexyl-1,2-diisocyanate,
cyclohexylene-1,4-diisocyanate, and diphenylmethanediisocyanates
such as 2,4'-diphenylmethanediisocyanate,
4,4'-diphenylmethanediisocyanate and mixtures thereof.
[0110] The amount of such crosslinking agents that can be used can
vary widely, and essentially any amount can be used. Generally,
however, the amount used will be about 1 to 50 wt. %, more
typically about 1 to 40 wt. %, about 3 to 40 wt. %, about 3 to 25
wt. %, about 5 to 40 wt. %, about 5 to 25 wt. %, or even about 5 to
12 wt. %, based on the dry weight of the hydrogel polymer that is
being crosslinked.
[0111] If a crosslinking agent is used, it can be added to the
other ingredients at any time during preparation of the inventive
self-suspending proppant. For example, in all embodiments of this
invention, it can be added to the proppant substrate particle
before it is combined with any other ingredient. Additionally or
alternatively, it can be added to each hydrogel polymer before it
is combined with the other ingredients forming the inventive
self-suspending proppant. Additionally or alternatively, it can
also be added to each hydrogel polymer after a coating layer made
from that hydrogel polymer is formed, thereby surface crosslinking
that coating layer.
[0112] For example, the outermost surface of the water-swellable
composite coating of the inventive self-suspending proppant can be
surface crosslinked by adding the crosslinking agent after all of
the hydrogel polymers have been added. Additionally and/or
alternatively, a crosslinking agent can be added after some but not
all of the hydrogel polymers have been added, thereby producing one
or more intermediate coating layers which themselves are surface
crosslinked. For example, a crosslinking agent can be added after
the addition of the first-applied hydrogel polymer is complete but
before the addition of the second-applied hydrogel polymer begins.
Additionally and/or alternatively, a crosslinking agent can be
added after the addition of the second-applied hydrogel polymer is
complete but before the addition of the third-applied hydrogel
polymer begins.
[0113] When a crosslinking agent is used, a catalyst for the
crosslinking agent can also be included, if desired. Examples of
suitable catalysts include acids, bases, amines and their
derivatives, imidazoles, amides, anhydrides, and the like. These
catalysts can be added together with the crosslinking agent or
separately. If added separately, they can be added at any time
during the preparation of the inventive self-suspending proppant,
in the same way as the crosslinking agent, as described above.
[0114] Another adhesion-promoting approach that can be used is
pretreating the proppant substrate particles with a suitable
adhesion promoter. For example, the proppant substrate particles
can be pretreated with a silane coupling agent before it is
combined with the hydrogel polymer forming the first coating. The
chemistry of silane coupling agents is highly developed, and those
skilled in the art should have no difficulty in choosing particular
silane coupling agents for use in particular embodiments of this
invention.
[0115] If desired, the silane coupling agent can be a reactive
silane coupling agent. As well understood in the art, reactive
silane coupling agents contain a functional group capable of
reacting with functional groups on the polymers to be coupled. In
this invention, therefore, the particular reactive silane coupling
agents used desirably contain functional groups capable of reacting
with the pendant hydroxyl, hydroxy methyl or other electronegative
groups of the hydrogel polymer forming the first coating layer of
the inventive proppants. Examples of such reactive silane coupling
agents include vinyl silanes such as vinyl trimethoxy silanes,
vinyl ethoxy silanes and other vinyl alkoxy silanes in which the
alkyl group independently have from 1 to 6 carbon atoms. Other
examples include reactive silane coupling agents which are based on
one or more of the following reactive groups: epoxy,
glycidyl/epoxy, allyl, and alkenyl.
[0116] Another type of adhesion promoter that can be used include
agents which provide a wetting/binding effect on the bond between
the proppant substrate particle and the hydrogel polymer forming
the first coating layer. Examples include reactive diluents, wax,
water, surfactants, polyols such as glycerol, ethylene glycol and
propylene glycol, various tackifiers such as waxes, glues,
polyvinyl acetate, ethylene vinyl acetate, ethylene methacrylate,
low density polyethylenes, maleic anhydride grafted polyolefins,
polyacrylamide and its blends/copolymerized derivatives, and
naturally occurring materials such as sugar syrups, gelatin, and
the like. Nonionic surfactants, especially ethoxylated nonionic
surfactants such as octylphenol ethoxylate, are especially
interesting.
[0117] Still another type of adhesion promoter that can be used is
the crosslinking agents mentioned above. In other words, one way
these crosslinking agents can be used is by pretreating the
proppant substrate particles with them before these particles are
mixed with the hydrogel polymer forming the first coating layer, as
described above.
Drying
[0118] In accordance with this invention, the intermediate product
produced when the water-swellable composite coating is formed on
the proppant substrate particles is dried to produce a mass of
free-flowing self-suspending proppants. Drying can be done without
application of heat, if desired, such as applying a vacuum.
Generally, however, drying can be done by heating the mixture at
temperatures as low as 40.degree. C. and high as 300.degree. C.,
for example. Generally, however, drying will be done at
temperatures above the boiling point of water such as, for example,
at >100.degree. C. to 300.degree. C., >100.degree. C. to
200.degree. C., 105.degree. C. to 150.degree. C., 110.degree. C. to
140.degree. C. or even 115.degree. C. to 125.degree. C.
[0119] Also, in those embodiments in which one or more of the
hydrogel polymers used is a starch which is heated for
gelatinization in an earlier process step, as described above,
drying will generally be done at drying temperatures which are
higher than the gelatinization temperature by at least 20.degree.
C., more typically at least 30.degree. C., at least 40.degree. C.,
or even at least 50.degree. C. In addition, in carrying out this
drying step, although the mixture being dried can be left
physically undisturbed until drying is completed, it is more
convenient to subject it to occasional mixing during drying, as
this helps keep the individual coated proppant particles from
sticking to one another, thereby minimizing particle clumping and
agglomeration.
[0120] One way that drying can be done is by placing the mixture in
a conventional oven maintained at a desired elevated temperature.
Under these conditions, drying will generally be completed in about
30 minutes to 24 hours, more typically about 45 minutes to 8 hours
or even 1 to 4 hours. Moreover, by occasionally mixing the mass
during this drying procedure, for example, once every 10 to 30
minutes or so, clumping and/or agglomeration of the coated proppant
will be largely avoided, resulting in a free-flowing mass of
proppants being produced.
[0121] Another convenient way of drying the mixture in accordance
with this invention is by using a fluidized bed dryer in which the
mixture is fluidized by an upwardly flowing column of heated air.
Fluidization causes individual coated proppant particles to
separate from one another, which not only avoids
clumping/agglomeration but also promotes rapid drying. Drying times
as short as 15 minutes, 10 minutes or even 5 minutes or less are
possible when fluidized bed dryers are used.
[0122] As a result of the manufacturing procedure described above,
a mass of individual, discrete starch-coated self-suspending
proppants can be produced. Although some clumping and agglomeration
might occur, these clumps and agglomerates can generally be broken
up by mild agitation. In addition, even if clumping and
agglomeration becomes more serious, application of moderate
pressure such as occurs with a mortar and pestle will usually be
sufficient to break up any agglomerates that have formed.
[0123] Properties
[0124] The inventive self-suspending proppant, optionally but
preferably, is free-flowing when dry. This means that any clumping
or agglomeration that might occur when this proppant is stored for
more than a few days can be broken up by moderate agitation. This
property is beneficial in connection with storage and shipment of
this proppant above ground, before it is combined with its aqueous
fracturing fluid.
[0125] When deposited in its aqueous fracturing fluid, the
inventive self-suspending proppant hydrates to achieve an effective
volumetric expansion which makes it more buoyant and hence
effectively self-suspending. In addition, it retains a significant
portion of this enhanced buoyancy even if it is exposed to hard or
salty water. Moreover, in some embodiments, it is also durable in
the sense that it retains a substantial degree of its
self-suspending character (i.e., its enhanced buoyancy) even after
being exposed to substantial shear forces.
[0126] This enhanced buoyancy can be quantitatively determined by a
Settled Bed Height Analytical Test carried out in the following
manner: 35 g of the proppant is mixed with 85 ml of the aqueous
liquid (e.g., preferably, water) to be tested in a glass bottle.
The bottle is vigorously shaken for 1 minute, after which bottle is
left to sit undisturbed for 5 minutes to allow the contents to
settle. The height of the bed formed by the hydrated, expanded
proppant is then measured using a digital caliper. This bed height
is then divided by the height of the bed formed by the uncoated
proppant substrate particle. The number obtained indicates the
factor (multiple) of the volumetric expansion.
[0127] In accordance with this invention, the inventive proppant is
desirably designed to exhibit a volumetric expansion, as determined
by this Settled Bed Height Analytical test when carried out using
simulated test waters having different levels of conductivities and
hardness, as described in Table 1, of .gtoreq..about.1.3,
.gtoreq..about.1.5, .gtoreq..about.1.75, .gtoreq..about.2,
.gtoreq..about.2.25, .gtoreq..about.2.5, .gtoreq..about.2.75,
.gtoreq..about.3, or even .gtoreq..about.3.5.
[0128] In this regard, it will be appreciated that a volumetric
expansion of 2 as determined by this test roughly corresponds to
cutting the effective density of the proppant in half For example,
if an inventive self-suspending proppant made from conventional
frac sand exhibits a volumetric expansion of 2 according to this
test, the effective density (i.e., the apparent specific gravity)
of this frac sand will have been reduced from about 2.65 g/cc to
about 1.4 g/cc. Persons skilled in the art will immediately
recognize that this significant decrease in density will have a
major positive effect on the buoyancy of the proppant obtained
which, in turn, helps proppant transport in hydraulic fracturing
applications tremendously, avoiding any significant proppant
settlement during this time.
[0129] In terms of maximum volumetric expansion, persons skilled in
the art will also recognize that there is a practical maximum to
the volumetric expansion the inventive proppant can achieve, which
will be determined by the particular type and amount of
hydrogel-forming polymers used in each application.
[0130] Another feature of the inventive proppant is that its
water-swellable composite coating rapidly swells when contacted
with water. In this context, "rapid swelling" will be understood to
mean that at least 80% of the ultimate volume increase that this
coating will exhibit is achieved within a reasonable time after
these proppants have been mixed with their aqueous fracturing
liquids. Generally, this will occur within 8 to 12 minutes of the
proppant being combined with its aqueous fracturing liquid,
although it can also occur within 30 minutes, within 20 minutes,
within 10 minutes, within 7.5 minutes, within 5 minutes, within 2.5
minutes or even within 1 minute of this time.
[0131] Still another feature of the inventive proppant is
durability or shear stability. In this regard, it will be
appreciated that proppants inherently experience significant shear
stress when they are used, not only from pumps which charge the
fracturing liquids containing these proppants downhole but also
from overcoming the inherent resistance to flow encountered
downhole due to friction, mechanical obstruction, sudden changes in
direction, etc. The water-soluble composite coatings of the
inventive self-suspending proppants, although inherently fragile
due to their hydrogel nature, nonetheless are durable enough to
resist these mechanical stresses and hence remain largely intact
(or at least associated with the substrate) until they reach their
ultimate use locations downhole.
[0132] For the purposes of this invention, coating durability can
be measured by a Shear Analytical Test in which the settled bed
height of a proppant is determined in the manner described above
after a mixture of 100 g of the proppant in 1 liter of water has
been subjected to shear mixing at a shear rate of about 511
s.sup.-1 for a suitable period of time, for example 5 or 10
minutes. The inventive self-suspending proppant desirably exhibits
a volumetric expansion, as determined by the above Settled Bed
Height Test, of at least about 1.3, more desirably about at least
about 1.5, at least about 1.6, at least about 1.75, at least about
2, at least about 2.25, at least about 2.5, at least about 2.75, at
least about 3, or even at least about 3.5 after being subjected to
the above shearing regimen for 5 minutes using ordinary tap water
as the test liquid. Inventive self-suspending proppants which
exhibit volumetric expansions of at least about 1.3, at least about
1.5, at least about 1.75, at least about 2, at least about 2.25, at
least about 2.5, at least about 2.75 or even at least about 3 after
having been subjected to the above shearing regimen for 10 minutes
using simulated test waters having different levels of
conductivities and hardness, as described in Table 1, are
especially interesting.
[0133] In addition to the above Shear Analytical Test, another
means for assessing coating durability is a Viscosity Measurement
Test in which the viscosity of the supernatant liquid that is
produced by the above Shear Analytical Test is measured after the
proppant has had a chance to settle. If the durability of a
particular proppant is insufficient, an excessive amount of its
water-swellable composite coating will come off and remain
dissolved or dispersed in the supernatant liquid. The extent to
which the viscosity of this liquid increases as a result of this
dissolved or dispersed coating is a measure of the durability of
the water-swellable composite coating. A viscosity of about 20 cPs
or more indicates a low coating durability. Desirably, the
viscosity of the supernatant liquid will be about 10 cPs or less,
more desirably about 5 cPs or less.
Working Examples
[0134] In order to more thoroughly describe this invention, the
following working examples are provided.
Materials
[0135] 20/40 mesh frac sand [0136] bPEI (Aldrich, St. Louis, Mo.)
[0137] FLOPAM EM533: high molecular weight, medium charge anionic
polyacrylamide inverse emulsion in petroleum distillate
(SNF--Riceboro, Ga.)) [0138] EM230: high molecular weight,
non-ionic polyacrylamide inverse emulsion in petroleum distillate
(SNF--Riceboro, Ga.) [0139] EM235: high molecular weight, very low
charge anionic polyacrylamide inverse emulsion in petroleum
distillate (SNF--Riceboro, Ga.) [0140] EM430: high molecular
weight, low charge anionic polyacrylamide inverse emulsion in
petroleum distillate (SNF--Riceboro, Ga.) [0141] EMR2545: very high
molecular weight, medium charge cationic polyacrylamide inverse
emulsion in petroleum distillate (SNF--Riceboro Ga.) [0142] EM1540
CT: high molecular weight, low charge cationic polyacrylamide
inverse emulsion in petroleum distillate (SNF--Riceboro, Ga.)
[0143] FB608: very high molecular weight, very high charge cationic
polyacrylamide inverse emulsion in petroleum distillate
(SNF--Riceboro, Ga.) [0144] FB 808: very high molecular weight,
very high charge cationic polyacrylamide inverse emulsion in
petroleum distillate (SNF--Riceboro, Ga.) [0145] Glycerol (US
Glycerin, Kalamazoo, Mich.) [0146] ethylene glycol [0147] polymeric
methylenediphenyldiisocyanate [0148]
Bis(3-dimethylaminopropyl)-n,n-dimethylpropanediamine (PolyCat 9:
Air Products, Allentown, Pa.) [0149] Potassium Chloride (The Home
Depot, Atlanta, Ga.) [0150] Calcium Chloride (Amazon-Home Brew
Ohio, Sandusky, Ohio) [0151] Calcium Chloride Dihydrate (Aldrich,
St. Louis, Mo.) [0152] Sodium Chloride (The Home Depot, Atlanta,
Ga.) [0153] Anhydrous sodium sulfate (Aldrich, St. Louis, Mo.)
[0154] magnesium chloride hexahydrate (Aldrich, St. Louis, Mo.)
[0155] Charge Master L340 Starch (Grain Processing Corporation,
Muscatine, Iowa)
TABLE-US-00001 [0155] TABLE 1 SNF Polymer Emulsion Information
Polymer Emulsion Charge Molecular Weight Structure* EM533 Medium
Anionic High EM230 Non-ionic High EM235 Very low Anionic High EM430
Low Anionic High EMR2545 Medium Cationic Very High Structured
EM1540 CT Low cationic High Linear FB 608 Very High Cationic Very
High Structured FB 808 Very High Cationic Very High Structured
*Where applicable
[0156] As used herein, the terms very high and high molecular
weight and very high, high, medium, low and very low anionic or
cationic charge have those meanings attributed to the polymers in
the art, such as the polymers commercially available as of the
filing date of this application by SNF Floerger,
http://snfus/wp-content/uploads/2014/07/SNF-Industrial-Product-Selection--
Guide-4-15-14.pdf. For example, FB608 has a cationic charge of
about 60 mole %. FB808 has a cationic charge of about 80 mole %.
Therefore, a very high cationic charge is meant to include polymers
having a charge of at least about 60 mole %. A high cationic charge
is meant to include polymers having a charge of between about 40
and 60 mole %. A medium cationic charge is meant to include
polymers having a charge of between about 20 and 40 mole %. A low
cationic charge is meant to include polymers having a charge of
between about 0.75 and 20 mole %. A high anionic charge is meant to
include polymers having a charge of at least about 60 mole %. A
medium anionic charge is meant to include polymers having a charge
of between about 20 and 60 mole %. A low anionic charge is meant to
include polymers having a charge of between about 3 and 20 mole
%.
Example 1: Two-Component Approach Vs. One-Component Approach
[0157] 90 g of 20/40 mesh sand was added to a FlackTek cup, along
with 0.09 g of a pre-coat containing 5 wt % ethylene glycol and 95
wt % water. The pre-coated sand was mixed at 850 RPM for 15
seconds. Separately, a coating composition was made up containing
10 wt % glycerol and 90 wt % of a commercially-available cationic
polyacrylamide inverse emulsion (FB608) containing approximately
equal amounts of a high molecular weight hydrogel-forming cationic
polyacrylamide copolymer, water and a hydrocarbon carrier liquid.
The weight ratio of hydrogel-forming polymer to glycerol in this
coating composition was about 3:1. 11.34 g of the aforementioned
cationic polyacrylamide inverse emulsion with 1.26 g glycerol was
added to the 20/40 mesh sand and mixed at 1500 RPM for 30 seconds.
2.5 g of a commercially-available liquid pMDI (polymeric
methylenediphenyldiisocyanate) covalent crosslinking agent was
subsequently added and then mixed in the same mixer at 850 RPM for
30 seconds. 1 g of a commercially available pMDI catalyst, known as
PolyCat 9, was added in and mixed the same way. The coated
proppants produced were oven dried for 30 minutes at 90.degree. C.
The sample was removed from the oven after 15 minutes and was
broken up by hand to allow for improved drying. The sample was then
sieved through an 18-mesh screen.
[0158] Another sample was prepared in the same way, except that an
anionic polyacrylamide emulsion (EM 533) was used instead of a
cationic one. This sample, along with the one including the
cationic emulsion, represent a single-component approach.
[0159] The last sample was prepared in the same way, except that
both anionic and cationic polymers (EM533 and FB608) were added
sequentially. 3.63 g of cationic polyacrylamide emulsion with 0.4 g
glycerol was added, mixed on 850 RPM for 15 seconds, followed by
0.91 g of anionic polyacrylamide emulsion with 0.1 g glycerol and
then an additional 15 seconds of mixing. Other chemicals were added
as described, and it was dried in the same manner.
[0160] Sand samples prepared were assessed for performance in a
settled bed height test. Settling heights were obtained by adding
(3 ppg) 54 g of the coated sand sample to 150 mL of water in a
small jar. Water that had 6,400 ppm of hardness and 29,000 ppm of
potassium chloride dissolved solids was used to prepare a hard
water replica for the purposes of these Examples. The hard water
replica recipe is shown in Table 2. The jar was inverted and gently
shaken a few times, and it was left to settle for five minutes. The
height of the sand layer after 5 minutes was measured with calipers
and compared against the height of the same amount of bare
sand.
TABLE-US-00002 TABLE 2 Hard Water Recipe #1 (6.4k hardness, 29.6k
ppm TDS) Salt Concentration (g/L) Potassium Chloride 22.5 Calcium
Chloride 7.1
[0161] There was 178% swelling in the sample with cationic
polyacrylamide only, 96% swelling in the sample with anionic only,
and the sample with the two-polymer approach had similar swelling
to the anionic sample with less than half the amount of polymer.
This example shows that the multi-component approach containing an
anionic and cationic polymer leads to a higher settled bed height
per amount of polymer added than just using a single component. In
addition, the clarity of the supernatant for the two-component
sample is much improved over both the cationic-only or anionic-only
samples.
Example 2: Order/Method of Addition
[0162] 1000 g of 20/40 mesh sand was added to a Kitchen Aid mixer,
along with 1 g of a pre-coat formulation containing 5 wt % ethylene
glycol and 95 wt % water. The pre-coated sand was stirred at the
lowest speed of the mixer for one minute. Separately, a coating
composition was made up containing 10 wt % glycerol and 90 wt % of
a commercially-available cationic polyacrylamide inverse emulsion
(FB608) containing approximately equal amounts of a high molecular
weight hydrogel-forming cationic polyacrylamide copolymer, water
and a hydrocarbon carrier liquid. The weight ratio of hydrogel
forming polymer to glycerol in this coating composition was about
3:1. 48.39 g of the aforementioned cationic polyacrylamide inverse
emulsion with 5.37 g glycerol was added to the 20/40 mesh sand. 12
g of a high molecular weight hydrogel-forming anionic
polyacrylamide (EM533) inverse emulsion with 1.33 g glycerol was
also added to the 20/40 mesh sand at the same time, and the mixture
was then stirred at the lowest speed of the mixer for 3.5 minutes.
The ratio of cationic to anionic polyacrylamide was 4:1. 2.5 g of a
commercially-available liquid pMDI (polymeric
methylenediphenyldiisocyanate) covalent crosslinking agent was
subsequently added. This was mixed in the same mixer on the lowest
setting for two minutes. 1 g of a commercially available pMDI
catalyst, known as PolyCat 9, was added in and mixed the same way
for 1.5 minutes. The coated proppants produced thereby were split
into two groups. One group was dried in an oven for 30 minutes at
90.degree. C., and the other group was dried in a fluidized bed
dryer for 7 minutes at 90.degree. C. on a speed setting of 42 rpm.
The sample in the oven was taken out after 15 minutes and was
broken up by hand to allow for improved drying. Both samples were
sieved through an 18-mesh screen.
[0163] Three more samples were prepared in the same way, except the
cationic and anionic polyacrylamide (FB608 and EM533) were added in
different manners. For one sample, the cationic polymer with
glycerol was added first, mixed in the manner described, and then
the anionic with glycerol was added and mixed as described. For the
other sample, the anionic polymer with glycerol was added first,
mixed in the manner described, and then the cationic with glycerol
was added and mixed as described. For the last sample, the anionic
and cationic polymer emulsions with glycerol were pre-mixed in the
same 4:1 ratio and then added in one step to the sand.
[0164] Sand samples prepared as described above were assessed for
performance in a settled bed height test. Settling heights were
obtained by adding 35 g of the coated sand sample to 84 mL of water
in a small jar. Two different types of water were used as
suspending fluid for these tests: Hard Water Recipe #2 and Hard
Water Recipe #3. These water recipes are shown in Tables 3 and
4.
[0165] Using these recipes to produce the suspending fluid for
settled bed height testing, the following experiments were
performed. First, the height of 35 g of coated sand is measured in
the graduated cylinder. The 35 g of coated sand is then added to 84
mL of water in the small jar. The jar was vigorously shaken for one
minute, left to settle for five minutes, and then inverted one more
time and poured into a 100 mL graduated cylinder. After five
minutes of settling in the graduated cylinder, the height of the
sand layer was measured. Settled bed heights from these tests are
reported in Table 5.
TABLE-US-00003 TABLE 3 Hard Water Recipe #2 (6.4k hardness, 29.6k
ppm TDS) Salt Concentration (g/L) Sodium Chloride 24 Calcium
Chloride Dihydrate 1.5 Anhydrous sodium sulfate 4.0 Magnesium
Chloride hexahydrate 10.8 Potassium Chloride 0.7
TABLE-US-00004 TABLE 4 Hard Water Recipe #3 (40k hardness, 350k ppm
TDS) Salt Concentration (g/L) Sodium Chloride 138.9 Calcium
Chloride Dihydrate 9.6 Anhydrous sodium sulfate 4.0 Magnesium
Chloride 67.7 hexahydrate Potassium Chloride 80.0
TABLE-US-00005 TABLE 5 Settled Bed Heights Hard Water Recipe #2
Hard Water Recipe #3 Dry Coated Settled Bed Swelling Dry Coated
Settled Bed Swelling Proppant Height Percentage Proppant Height
Percentage (mm) (mm) (%) (mm) (mm) (%) Simultaneous Fluidized 22 42
91 50* Addition Bed Oven* N/A N/A N/A N/A N/A 40* Anionic Fluidized
N/A N/A N/A N/A N/A 90* Before Bed* Cationic Oven* N/A N/A N/A N/A
N/A 40-50* Cationic Fluidized 23 39 70 23 41 78 Before Bed Anionic
Premixed Fluidized 22 54.5 148 22 46 109 Bed *Indicates measurement
by visually estimating the swelling percentage.
[0166] This is a process that does not involve emptying contents
into a graduated cylinder. Instead, bed height is estimated from
the height in the jar after 1 minute of shaking and 5 minutes of
settling.
[0167] As additional findings, we observed visually that oven
drying produced product that was more prone to caking and had less
flowability. Fluidized bed drying produced higher settled bed
heights. These experiments showed that a premixed version of
cationic and anionic polyacrylamide emulsion delivered the highest
settled bed heights.
Example 3: Differing Ratios
[0168] Samples were created in the same manner and with the same
materials as discussed in Example 1. This time, the polymer
emulsion addition contained both the anionic and cationic polymers
(EM533 and FB608), but the cationic emulsion was added first, mixed
in the FlackTek mixer, and then the anionic emulsion was added in,
mixed in the mixer. Following this, the other chemicals were added
and drying procedures were performed as described in Example 1.
Four different samples were created in this manner, with varying
ratios of cationic to anionic polymer emulsion but with the same
total amount of emulsion added; also, two other samples were
created as controls, with one having only the cationic emulsion and
the other having only the anionic emulsion. Settled bed heights
were measured in the same manner as described in Example 1, and
they were remeasured two days after shaking. Table 6 shows the
makeup of each sample and the settled bed height results, using
Hard Water Recipe #1 that was described in Table 2 of Example
1.
TABLE-US-00006 TABLE 6 Settled Bed Heights for Varying Ratios of
Cationic to Anionic Polymer Emulsions Settled Bed Settled Bed
Height After Two Height Change % Cationic/ Dry Bare Sand Settled
Bed Swelling Days of Settling After Two Days % Anionic Height (mm)
Height (mm) Percentage (%) (mm) (mm) 60/40 11.5 24.02 109 18.54
5.48 70/30 20.28 76 18.88 1.40 80/20 22.56 96 18.72 3.84 90/10
21.16 84 19.47 1.69 100/0 22.26 94 18.91 3.35 0/100 17.88 55 --
--
[0169] The sample with 100% cationic emulsion had the most turbid
water, consistent with the polymer coating shedding from the sand.
This finding suggests that, although swelling is similar, the
multi-component approach performs without as much polymer shedding.
The anionic polymer alone does not perform as well in several
regards.
[0170] In a second test, more ratios of cationic to anionic polymer
emulsions were tested, using the KitchenAid process and settled bed
height testing process outlined in Example 2, and using the hard
water recipes from Example 2. Results showed that the 80/20 ratio
with premixed polymer produced superior results to other ratios
with cationic and anionic emulsions added sequentially. Tables 7
and 8 and FIG. 2 illustrate these results. Because Samples 1 and 4
produced the best qualitative results (as shown in FIG. 1), these
were emptied into graduated cylinders for additional
measurement.
TABLE-US-00007 TABLE 7 Settled Bed Heights for Varying Ratios of
Cationic to Anionic Polymer Emulsions in Hard Water Recipe #1
(using the KitchenAid process from Example 2) Dry Coated Settled
Proppant Bed Swelling Height Height Percentage Sample % Cationic/%
Anionic (mm) (mm) (%) 1 80/20 premixed 22 54.5 148 2 50/50
sequential addition* N/A N/A 60* 3 70/30 sequential addition* N/A
N/A 90* 4 92.5/7.5 sequential addition 24 70 192 *Indicates
measurement by visually estimating the swelling percentage.
[0171] This is a process that does not involve emptying contents
into a graduated cylinder. Instead, bed height is estimated from
the height in the jar after 1 minute of shaking and 5 minutes of
settling.
[0172] The bed height of sample 4 decreased to 64 mL after ten
minutes and to 62 after 15 minutes, whereas the bed height of
sample 1 stayed more constant over a 15-minute timeframe.
TABLE-US-00008 TABLE 8 Settled Bed Heights for Varying Ratios of
Cationic to Anionic Polymer Emulsions in Hard Water Recipe #3 with
Alternative Process Dry Coated Proppant Swelling Height Settled Bed
Percentage Sample % Cationic/% Anionic (mm) Height (mm) (%) 1 80/20
premixed 22 46 109 4 92.5/7.5 sequential 24 44 83 addition
Example 4: Different Polymers--bPEI
[0173] Samples were created in the same manner and with the same
materials as described in Example 1. This time, the polymer
emulsion addition contained either an anionic or cationic polymer
(outlined in Table 9) and a commercially available low molecular
weight branched polyethyleneimine (bPEI) (from Sigma Aldrich, with
a weight average molecular weight of 2000, provided as a 50%
solution in water). This bPEI was added first mixed in the FlackTek
mixer for 15 seconds at 850 RPM, and then the emulsion was added
in, mixed for 15 seconds at 850 RPM, and the other chemicals were
added as described previously, and drying was done for 15 minutes
at 90.degree. C.
[0174] Varying amount of bPEI were added. Settled bed heights were
obtained as described in Example 1, using Hard Water Recipe #1.
Table 9 shows the makeup of each sample and its swelling
percentage. Certain of these samples are illustrated in FIG.
3-5.
TABLE-US-00009 TABLE 9 Sample Makeup and Settled Bed Heights for
Multi-Component Testing with bPEI bPEI Polymer Amount Polymer
Amount Swelling Sample (g) bPEI Addition Layer (g) Percentage (%)
30 0.9 Before EM1540 2.52 50 31 Polymer EM235 59 32 Layer EM230 59
33 1.8 EM1540 56 34 EM235 -- 35 EM230 61 47 0.685 Premixed into
EM1540 1.835 47 48 Polymer EM235 41 49 EM230 53
[0175] We observed that the multi-component system swelled somewhat
in hard water, but it did not appear to offer much improvement in
settled bed height as compared to that of a single polymer emulsion
alone. The addition of bPEI did not appear to influence swelling
significantly. Turbidity in some of the samples suggested that
there was polymer shedding to various extents.
Example 5: Different Polymers--Starches
[0176] Samples were created in the same manner and with the same
materials as discussed in Example 1. This time, the polymer
emulsion addition contained either an anionic or cationic polymer
(outlined in Table 10) and a commercially available, pregelatinized
cationic starch. As before, the previously discussed ethylene
glycol and water mixture was added first to 90 g of sand, mixed as
described in Example 1, and then the starch was added. The starch
used was Charge Master L430. The starch was then added to the sand
mixed in the FlackTek mixer for 15 seconds at 850 RPM, and dried
for 20 minutes at 90.degree. C. Then the test emulsion was added
in, mixed for 15 seconds at 850 RPM; other chemicals were added as
described in Example 1, and drying was done for 20 minutes at
90.degree. C.
[0177] Varying amounts of the pregelatinized cationic starch were
added. Settled bed heights were measured in the same manner, using
the hard water recipe described in Example 1. Table 10 shows the
makeup of each sample and its swelling percentage.
TABLE-US-00010 TABLE 10 Sample Makeup and Settled Bed Heights for
Multi-Component Testing with Starch Starch Polymer Amount Starch
Polymer Amount Swelling Sample (g) Addition Layer (g) Percentage
(%) 14 1.35 Before EM533 3.00 26 15 Polymer EM1540 59 16 Layer EMR
2545 50 17 EM235 47 18 EM230 50 19 EM430 41 20 2.70 EM 533 2.52 44
21 EM1540 53 22 EM1540 41 23 EMR2545 53 24 EM235 44 25 EM230 59 26
EM430 47 27 EM533 38
[0178] We observed that the multi-component system swelled somewhat
in hard water, but it did not appear to offer much improvement in
settled bed height as compared to that of a polymer emulsion alone.
The amount of starch added did not appear to make a large
difference in the resulting swelling. The samples containing
emulsions with anionic charges, such as samples containing EM533,
EM235, and EM430 generally had lower amounts of swelling. We also
observed that the all proppant samples containing starch were very
sticky, and large clumps formed.
Example 6: Removing Glycerol
[0179] Glycerol was removed for this set of tests in order to
address concerns about humidity tolerance and sample clumping.
[0180] Samples were created in the KitchenAid mixer in the same
manner and with the same materials as discussed in Example 2 (and
oven dried only). This time, the polymer emulsions tested were
FB608, FB808, and EM533, and they were added to the sand without
glycerol, but all with other chemical amounts were the same, using
a 4:1 cationic to anionic emulsion ratio. Both glycerol-free
samples were created with the cationic and anionic emulsions
premixed. Table 11 shows the settled bed height results. Settled
bed height was tested by using the hard water recipe from Example 1
and the test method described in Example 2, except the contents of
the jar were poured into the graduated cylinder immediately after
shaking and then left to settle in the graduated cylinder for five
minutes. Once glycerol was removed and testing was run, a larger
difference in premixing vs sequential addition and in varying
ratios was seen as well.
TABLE-US-00011 TABLE 11 Settled Bed Heights for Multi-Component
Testing without Glycerol Dry Proppant Swelling Bed Settled Bed
Percentage Sample Height (mm) Height (mm) (%) With Glycerol (with
FB608) 27.0 42.0 55.6 Without Glycerol (with FB 608) 25.5 45.0 76.5
Without Glycerol (with FB 808) 26 49 88.5
[0181] The same amount of total polymer emulsion was used with a
70/30 ratio of cationic to anionic emulsion (they were again
premixed) but with varying amounts of glycerol. Results are shown
in Table 12.
TABLE-US-00012 TABLE 12 Settled Bed Heights for Multi-Component
Testing with Varying Amounts of Glycerol Swelling Dry Proppant Bed
Settled Bed Percentage Sample Height (mm) Height (mm) (%) 0%
Glycerol (with FB808) 26.0 47.0 80.8 3% Glycerol (with FB608) 26.0
47.0 80.8 5% Glycerol (with FB608) 26.5 44.5 67.9
[0182] Less or a lack of glycerol showed a significant improvement
in swelling. The FB808 showed improved results over the FB608 that
was used in Examples 1-3. The FB608 and FB808 cationic
polyacrylamide emulsions have similar properties. The largest
difference is that FB808 has a higher viscosity than FB608. Further
testing was completed to show that, without glycerol, premixing
emulsions and an 80/20 cationic to anionic emulsion ratio resulted
in higher amounts of swelling (Tables 13 and 14).
TABLE-US-00013 TABLE 13 Settled Bed Heights for Multi-Component
Testing without Glycerol (with EM533 and FB 608 in KitchenAid) Dry
Proppant Swelling Bed Settled Bed Percentage Sample Height (mm)
Height (mm) (%) 80/20 Ratio with Premixed 25.5 45.0 76.5 Emulsion
80/20 Ratio with cationic added, 26.5 42.5 60.4 mixed, and then
anionic added
TABLE-US-00014 TABLE 14 Settled Bed Heights for Multi-Component
Testing without Glycerol (with EM533 and FB 808 in FlackTek) Dry
Proppant Swelling Bed Settled Bed Percentage Sample Height (mm)
Height (mm) (%) 80/20 Ratio with Premixed 26.0 49.0 88.5 Emulsion
70/30 Ratio with Premixed 26.0 47.0 80.8 Emulsion
Example 7
[0183] In these examples, self-suspending proppants made in
accordance with this invention were tested for their ability to
swell when exposed to different simulated test waters. Test waters
(TW) 4, 5, 6 and 7 were formulated with varying amounts of CaCl,
MgCl, NaCl and KCl to mimic the different types of aqueous liquid
generally found in hydraulic fracturing. Test water 1 was
formulated to simulate sea water. The properties of these test
waters are set forth in the following Table 15:
TABLE-US-00015 TABLE 15 Properties of Test Waters (TW) Properties
of Each Test Water Fresh Property Water TW 4 TW 5 TW 6 TW 7 pH 6.5
5.8 5.7 5.8 6.2 Conductivity, 295 19,200 115,200 242,000 501,000
.mu.S Hardness, ppm 120 6,400 6,400 6,400 40,000 TDS*, ppm
<1,000 29,600 69,500 136,00 350,000 *Total Dissolved Solids
Example 8-Anionic PAM/Cationic Starch Hybrid
[0184] 1000 g of sand was added to the mixing bowl of a commercial
Kitchen Aid mixer. 1 g of a 5% PEG-DGE (polyethylene glycol
diglycidyl ether) solution in ethylene glycol:water (5:95) was then
added, and the mixture obtained was mixed for an additional 1
minute at speed setting 2 of the machine (about 70 rpm).
[0185] 25.2 g of a commercially available anionic polyacrylamide
inverse emulsion containing approximately one third by weight
organic solvent, one third water and one third of an anionic
polyacrylamide polymer made by copolymerizing acrylamide and
acrylic acid was used to form the first coating of the
self-suspending proppants of this example. This was done by
thoroughly mixing this anionic polyacrylamide inverse emulsion with
2.8 of glycerol and then adding the mixture so formed to the
treated sand in the mixing bowl, with further mixing for 3.5
minutes at a speed setting of 2.
[0186] 30 g of a 40% aqueous dispersion of a commercially available
cationic starch was then used to form the second hydrogel polymer
coating of the self-suspending proppants of this example. This was
done by adding this starch dispersion to the contents of the mixing
bowl, followed by adding 6.4 g of PPGDGE (polypropylene glycol
diglycidyl ether) as a crosslinking agent for the starch and 16 g
of SM NaOH as a catalyst for the PPGDGE, with continued mixing for
an additional 5 minutes at speed setting 3 of the machine. The
mixture so obtained was then transferred to a fluidized bed dryer
and dried for not more than 5 minutes at 90.degree. C. at 38
rpm.
[0187] A number of different runs were made including a control run
in which no cationic starch was used. In some cases the partially
dried mixture obtained above was transferred back to the Kitchen
Aid mixing bowl and further mixed with 2.5 g of a p-MDI covalent
crosslinking agent for 2 minutes at speed 2, followed by 2 g of 20%
aqueous solution of a tertiary amine catalyst for the p-MDI and
mixed for 1.5 minutes at speed 2. In all cases the mixture was
transferred into an aluminum foil tray and further dried for 30
minutes at 90.degree. C. in a convection oven to obtain a free
flowing coated proppant. Several coatings were made using varying
amounts of anionic polyacrylamide emulsion and cationic starch
dispersion, keeping all other ingredients the same.
[0188] Proppants obtained were then tested using the Settled Bed
Height analytical test described above to determine their ability
to swell when contacted with the test waters described in Table
15.
[0189] The composition of each proppant tested and the results
obtained are shown in the following Table 16:
TABLE-US-00016 Proppant Composition, wt % (dry), based on weight of
sand substrate Control Run 1 Run 2 Run 3 Run 4 Run 5 Anionic
Polyacrylamide 0.91 0.91 0.45 1.19 1.44 1.44 Cationic starch 0 1.20
2.00 1.60 1.32 1.32 Total hydrogel 0.91 2.11 2.45 2.79 2.76 2.76
PPGDGE 0 0.68 0.68 0.68 0.68 0.68 NaOH 0 0.32 0.32 0.32 0.32 0.32
pMDI 0.25 0.25 0.25 0.25 0 0.25 catalyst 0.04 0.04 0.04 0.04 0 0.04
Performance Testing--Swelling % Fresh water 400 400 400 400 400 400
TW 4 10 90 70-80 110-120 100 100 TW 5 10 90 70-80 110-120 100 100
TW 6 10 90 70-80 110-120 100 100 TW 7 10 90 70-80 110-120 100
100
[0190] As can be seen from Table 16, the proppants exhibited
substantial swelling when exposed to fresh water. However the
control proppant, which was made with no cationic starch, exhibited
very little swelling when exposed to all four different test
waters. On the other hand, the inventive proppants exhibited
substantial swelling in different test waters, even though they
were made with comparatively little amounts of hydrogel polymer in
total.
Example 9 Cationic PAM/Anionic Starch Hybrid
[0191] Example 8 was repeated except that a commercially available
cationic polyacrylamide inverse emulsion containing approximately
one third polymer, one third organic solvent and one third water
was used to form the first coating on the sand substrate particles,
while a 40% aqueous dispersion of a commercially available anionic
starch was used to form the second hydrogel polymer coating.
[0192] The composition of proppants were tested and the results
obtained are shown in the following Table 17:
TABLE-US-00017 Proppant Composition, wt % (dry), based on weight of
sand substrate Run 1 Run 2 Run 3 Run 4 Run 5 Cationic 0.63 0.31
0.83 1.00 1.00 Polyacrylamide Anionic starch 1.20 2.00 1.60 1.32
1.32 Total hydrogel 1.83 2.31 2.43 2.32 2.32 PPGDGE 0.68 0.68 0.68
0.68 0.68 NaOH 0.32 0.32 0.32 0.32 0.32 pMDI 0.25 0.25 0.25 0.25 0
catalyst 0.04 0.04 0.04 0.04 0 Performance Testing--Swelling % TW 4
60 70-80 80 90 90 TW 5 60 70-80 80 90 90 TW 6 60 70-80 80 90 90 TW
7 60 70-80 80 90 90
[0193] As can be seen from Table 17, the inventive proppants
exhibited substantial swelling in these different test waters, even
though they were also made with comparatively little amounts of
hydrogel polymer intotal.
Example 10 Anionic PAM/Cationic PAM Hybrid
[0194] Examples 8 and 9 were repeated, except that the first
hydrogel polymer coating was formed from a commercially available
anionic polyacrylamide inverse emulsion while the second hydrogel
polymer coating was formed from a commercially available cationic
polyacrylamide inverse emulsion. Two different commercially
available anionic polyacrylamide inverse emulsions were used for
this purpose, both of which were formulated from polyacrylamide
polymers made by copolymerizing acrylamide with acrylic acid or an
acrylic acid salt. Similarly, two different commercially available
cationic polyacrylamide emulsions were used for this purpose.
[0195] The composition of the proppants were tested and the results
obtained are shown in the following Table 18:
TABLE-US-00018 Proppant Composition, wt % (dry), based on weight of
sand substrate Run 1 Run 2 Run 3 Run 4 Run 5 1st Cationic 0 0 0.72
1.08 1.08 Polyacrylamide 2nd Cationic 0.99 0.99 0 0 0
Polyacrylamide 1st Anionic 0 0 0 0 0.83 Polyacrylamide 2nd Anionic
0.99 0.66 0.50 0.50 0 Polyacrylamide Total hydrogel 1.98 1.65 1.22
1.58 1.91 pMDI 0.25 0.25 0.25 0.25 0.25 Catalyst 0.4 0.4 0.4 0.4
0.4 Performance Testing--Swelling % TW 4 80 40 30 40 70 TW 5 80 40
30 40 70 TW 6 80 40 30 40 70 TW 7 80 40 30 40 70
[0196] As can be seen from Table 18, all five of the inventive
proppants exhibited at least some significant degree of swelling in
these different test waters, even though they were made with very
small amounts of hydrogel polymer in total.
Example 11 Hydrolyzed Anionic PAM/Cationic PAM Hybrid
[0197] 1000 g of sand was added to the mixing bowl of a commercial
Kitchen Aid mixer. In some runs, 2 g of a 5% PEG-DGE (polyethylene
glycol diglycidyl ether) solution in ethylene glycol:water (5:95)
was then added, followed by mixing for an additional 1 minute at
speed setting 2 of the machine (about 70 rpm). In other runs, 1 g
of a glycol:water (5:95) mixture was used for this purpose.
[0198] A suitable amount, for example, 12.1 g, of a commercially
available anionic polyacrylamide inverse emulsion was mixed with a
suitable amount, for example, 48.3 g, of a commercially available
cationic polyacrylamide inverse emulsion. The mixture so obtained
was then added to the mixing bowl containing the previously treated
sand, with continued mixing for an additional 3.5 minutes at a
speed setting of 2. 2.5 g of a p-MDI covalent crosslinking agent
was then added with mixing for an additional 2 minutes at speed
setting 2, followed by the addition of 2 g of a 20% aqueous
solution of a tertiary amine catalyst for the p-MDI, with mixing
for an additional 1.5 minutes at speed setting 2. In all cases the
mixture was transferred to a fluid bed dryer and further dried for
7 to 10 minutes at 90.degree. C. and 38 rpm, to obtain a dry, free
flowing coated proppant.
[0199] Five different self-suspending proppants were made using
varying amounts of anionic and cationic polyacrylamide emulsions,
keeping all other ingredients the same. In Runs 1, 2, 3 and 5, the
anionic polyacrylamides used were hydrolyzed polyacrylamide having
different degrees of hydrolysis (charge density) ranging from 10 to
90 mole %, more typically 10 to 60 mole %, 15 to 50 mole %, or even
20 to 40 mole %. Meanwhile, in Run 4 the anionic polyacrylamides
used was made by copolymerization of acrylamide and acrylic acid or
an acrylic acid salt.
[0200] The proppants obtained were then tested using the Settled
Bed Height analytical test described above to determine their
ability to swell when contacted with the test water described
above.
[0201] The composition of each proppant tested and the results
obtained are shown in the following Table 19:
TABLE-US-00019 Proppant Composition, wt % (dry), based on weight of
sand substrate Run 1 Run 2 Run 3 Run 4 Run 5 1st Cationic 2.46 2.05
1.85 1.54 0 Polyacrylamide 2nd Cationic 0 0 0 0 2.09 Polyacrylamide
1st Anionic 0.45 0.37 0 0 0 Polyacrylamide 2nd Anionic 0 0 0.72 0 0
Polyacrylamide 3rd Anionic 0 0 0 1.42 0 Polyacrylamide 4th Anionic
0 0 0 0 0.57 Polyacrylamide Total hydrogel 2.91 2.43 2.57 2.96 2.66
pMDI 0.25 0.25 0.25 0.25 0.25 catalyst 0.02 0.02 0.02 0.02 0.02
Performance Testing--Swelling % TW4 175 145 145 140 125 TW7 150 130
115 96 125
[0202] As can be seen from Table 19, the inventive proppants
exhibited a significant degree of swelling in different test
waters, even though they were made with very small amounts of
hydrogel polymer in total. In addition, by comparing Run 4 with the
Runs 1, 2, 3 and 5, it can be seen that the inventive
self-suspending proppants made with hydrolyzed anionic
polyacrylamide exhibit exceptionally good tolerance to waters with
very high salt contents.
Example 12 Anionic PAM/Nonionic Starch
[0203] 1000 g of 50.degree. C. pre-heated sand was added to the
mixing bowl of a commercial Kitchen Aid mixer. In some runs, 2 g of
a 5% PEGDGE (polyethylene glycol diglycidyl ether) solution in
ethylene glycol:water (5:95) was then added, followed by mixing for
an additional 1 minute at speed setting 2 of the machine (about 70
rpm). In other runs, 1 g of a glycol:water (5:95) mixture was used
for this purpose.
[0204] A suitable amount of the same anionic polyacrylamide inverse
emulsion used in Example 8 was added to the mixing bowl, after
which a suitable amount of a commercially available nonionic
starch, in particular a pre-crosslinked, cold water-swellable
modified waxy maize starch, was added and the mixture so obtained
mixed for an additional 3.5 minutes at speed setting 2 of the
machine. In some runs, the anionic polyacrylamide inverse emulsion
contained 10 wt. % glycerol based on the combined weight of the
glycerol and emulsion, while in other runs it did not. In addition,
in some runs, the pre-crosslinked, cold water-swellable maize
starch was added in powder form, as received from the manufacturer,
while in other runs it was added in the form of a 60 wt. %
dispersion in either IPA (isopropyl alcohol) or a water-white
commercially available isoparaffinic organic solvent (IsoparG).
[0205] Then 1.25 g of a p-MDI crosslinking agent was added with
continuous mixing for 2 minutes at speed setting 2, followed by the
addition of 1 g of a 20% aqueous solution of a tertiary amine
catalyst for the p-MDI, followed by additional mixing for 1.5
minute at speed setting 2. Various amounts of water were then
sprayed into the mixing bowl, following which the mixture was
transferred to a fluidized bed dryer and dried for 5 minutes at
90.degree. C. at 38 rpm to obtain a free flowing coated
proppant.
[0206] Several coatings were made using varying amounts of
different components to obtain optimum performance.
[0207] The proppants obtained were then tested using the Settled
Bed Height analytical test described above to determine their
ability to swell when contacted with the test waters. The
compositions and the results obtained are shown in the following
Table 20:
TABLE-US-00020 Proppant Comp, wt % (dry), based on weight of sand
substrate Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Pretreat Sand w PEGDE
No No Yes Yes Yes No EG in Anionic PAM Emulsion Yes Yes No No No
Yes Anionic PAM, wt, % 1.2 1.2 1.2 1.2 1.2 1.2 Nonionic starch, wt,
% 3.1 3.1 3.1 3.1 3.1 3.1 Total Hydrogel, wt. % 4.3 4.3 4.3 4.3 4.3
4.3 Form of Nonionic Starch IPA disp Iso-G disp Iso-G disp IPA disp
powder IPA disp Amount of Water Spray, g 12.88 12.88 12.88 12.88
13.92 12.88 % Swelling, TW 4 155 120 120 155 125 155 % Swelling, TW
7 115 100 100 130 90 105
[0208] As can be seen from Table 20, the inventive proppants
exhibited a significant degree of swelling in different test
waters, even though they were made with small amounts of hydrogel
polymer in total.
Example 13 Anionic PAM/Cationic PAM/Nonionic Modified Starch
Hybrid
[0209] Example 11 was repeated, except that 5-100% wt. % of a
nonionic starch, based on the combined weights of the
anionic/cationic polyacrylamide mixture used, was also used to make
the hydrogel coating of these proppants. In some runs, the nonionic
starch was premixed with a mixture of the anionic and cationic
polyacrylamide dispersions. In other runs, each of these hydrogel
polymers was separately added so that three separate hydrogel
coating layers were formed, with the nonionic starch coating layer
comprising either the first, second or third coating layer. Also,
in some instances, the nonionic modified starch was added in the
form of a powder, while in other instances it was added in the form
of an aqueous dispersion. In addition, in those instances in which
the nonionic modified starch was added in the form of a powder,
various amounts of water were then sprayed into the mixing bowl, as
described in the above Example 12.
[0210] The composition of each proppant tested and the results
obtained are shown in the following Table 21:
TABLE-US-00021 Proppant Composition, wt % (dry), based on weight of
sand substrate Run 1 Run 2 Run 3 Run 4 Run 5 Cationic 1.51 1.51
0.95 1.51 0.95 Polyacrylamide Anionic 0.29 0.29 0.18 0.29 0.18
Polyacrylamide Nonionic Modified 0.21 0.71 2.52 0.10 2.52 Starch
Total hydrogel 2.01 2.51 3.65 1.90 3.65 pMDI 0.12 0.12 0.12 0.12
0.12 catalyst 0.02 0.02 0.02 0.02 0.02 Form of Starch Powder Powder
Powder Aq. disp. Powder Amount of Water 1.65 5.51 9.84 0 9.84
Spray, g Performance Testing--Swelling % Swelling % in TW4 125 125
115 140 110 Swelling % in TW7 110 110 105 110 105
[0211] As can be seen from Table 21, the inventive proppants
exhibited a significant degree of swelling in different test
waters, even though they were made with relatively small amounts of
hydrogel polymer in total.
Example 14 Cationic PAM/Nonionic Modified Starch Hybrid
[0212] Example 8 was repeated except that a commercially available
cationic polyacrylamide inverse emulsion containing approximately
one third polymer, one third organic solvent and one third water
was used to form the first coating on the sand substrate particles,
while an aqueous dispersion of a commercially available nonionic
modified starch was used to form the second hydrogel polymer
coating in some runs (Run 2 through Run 4), while another nonionic
modified starch aqueous dispersion or powder was used to form the
second hydrogel layer in other runs (Run 5 through Run 7). In those
instances in which a nonionic modified nonionic starch in powder
form was used, the powder was added after the first coating or was
mixed with the cationic hydrogel polymer first and then coated onto
substrate. One experiment was also carried out without any nonionic
starch coating (Run 1).
[0213] The composition of each proppant tested and the results
obtained are shown in the following Table 22:
TABLE-US-00022 Proppant Composition, wt % (dry), based on weight of
sand substrate Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Run 7 Cationic
Polyacrylamide 2.3 2.3 2.6 2.6 2.3 2.6 2.6 Nonionic modified starch
0 1.6 2.3 3.3 0.21 1.5 2.4 Total hydrogel 2.3 3.9 4.9 5.9 2.51 4.1
5.0 PPGDGE 0 0.68 0.68 0.68 0 0 0 NaOH (5M) 0 1 1 1 0 0 0 pMDI 0.25
0.25 0.25 0.25 0.25 0.25 0.25 Catalyst 0.02 0.02 0.02 0.02 0.02
0.02 0.02 Form of the Starch N/A disp disp. disp. Powder Powder
disp. Performance Testing--Swelling % TW 4 160 85 100 120 130 140
160 TW 7 130 75 90 110 120 130 150
[0214] As can be seen from Table 8, all of the inventive proppants
exhibited varying degrees of swelling in different test waters,
even though they were also made with comparatively little amounts
of hydrogel polymer in total.
[0215] Although only a few embodiments of this invention have been
described above, it should be appreciated that many modifications
can be made without departing from the spirit and scope of the
invention. All such modifications are intended to be included
within the scope of this invention, which is to be limited only by
the following claims.
* * * * *
References